Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions

Modern Biocatalysis Stereoselective and Environmentally Friendly Reactions Edited by Wolf-Dieter Fessner and Thorleif A...

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Modern Biocatalysis Stereoselective and Environmentally Friendly Reactions Edited by Wolf-Dieter Fessner and Thorleif Anthonsen

Modern Biocatalysis Edited by Wolf-Dieter Fessner and Thorleif Anthonsen

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Modern Biocatalysis Stereoselective and Environmentally Friendly Reactions Edited by Wolf-Dieter Fessner and Thorleif Anthonsen

The Editors Prof. Dr. Wolf-Dieter Fessner TU Darmstadt, Inst. f. Organische Chemie und Biochemie Petersenstr. 22 64287 Darmstadt Prof. Dr. Thorleif Anthonsen Norwegian Unvi. of Science & Technology, Dept. of Chemistry Hogskoleringen 5 7491 Trondheim Norwegen Cover Illustration: Background: Modified part of the “Biochemical Pathways Chart”, 4th edition, by courtesy of F. Hoffmann-La Roche Ltd. Protein structure taken from the RSCB Protein Data Base and modified: PDB ID: 1ADO N. Blom, J. Sygusch, Product binding and role of the C-terminal region in class I D-fructose 1,6bisphosphate aldolase, Nat. Struct. Biol. 4 (1997), 36–39

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Typesetting SNP Best-set Typesetter Ltd., Hong Kong Printing Strauss GmbH, Mörlenbach Binding Litges & Dopf GmbH, Heppenheim Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32071-4

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Contents Preface

XV

List of Contributors 1 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5 1.4 1.4.1 1.4.2 1.4.3 1.5 1.6

2 2.1 2.2 2.2.1 2.2.2

XVII

Fluorescence Assays for Biotransformations 1 Jean-Louis Reymond Introduction 1 Alcohol Dehydrogenases (ADHs) and Aldolases 2 Chiral Fluorogenic ADH Substrates 2 Fluorogenic Aldolase Probes 3 Transaldolases and Transketolases 4 Enolase Probe 4 Lipases and Esterases 5 Assays on Solid Support 6 The Clips-O Substrates with Periodate 8 Esters of Fluorogenic Cyanohydrins and Hydroxyketones Fluorogenic Acyloxymethyl Ethers 10 FRET-Lipase Probes 11 Other Hydrolases 11 Epoxide Hydrolases 11 Amidases and Proteases 13 Phosphatases 14 Baeyer–Villigerases 15 Conclusion 15 Acknowledgment 16 References 16 Immobilization as a Tool for Improving Enzymes Ulf Hanefeld Introduction 21 Adsorption/Electrostatic Interactions 22 Van der Waals Interactions 22 Hydrogen Bonds 25

Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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9

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Contents

2.2.3 2.3 2.4 2.5

Ionic Interactions 27 Encapsulation 31 Covalent Binding/Cross-linking Conclusion 38 Acknowledgments 38 References 39

3

Continuous-flow Microchannel Reactors with Surface-immobilized Biocatalysts 43 Malene S. Thomsen and Bernd Nidetzky Introduction 43 Biocatalytic Synthesis Using Microreaction Technology with Free and Immobilized Enzymes 44 Novel Microfluidic Immobilized Enzyme Reactors 45 Microreactor Design 45 Enzyme Immobilization 47 Enzymatic Hydrolysis of Lactose 48 Catalytic Effectiveness of Immobilized CelB 48 Continuous Conversion of Lactose 48 Biocatalytic Process Intensification Using Microreaction Technology 50 Conclusions and Outlook 51 Acknowledgements 52 References 52

3.1 3.2 3.3 3.3.1 3.3.2 3.4 3.4.1 3.4.2 3.5 3.6

4 4.1 4.2 4.3 4.4 4.5

5

5.1 5.2 5.3 5.4

33

Activity and Stability of Proteases in Hydrophilic Solvents 55 Lars Haastrup Pedersen, Sinthuwat Ritthitham, and Morten Kristensen Introduction 55 Activity and Selectivity of Proteases in Synthesis of Carbohydrate Fatty Acid Esters 56 Enzyme Stability and Conformation 59 Solvent Engineering 63 Conclusion 64 References 65 Importance of Enzyme Formulation for the Activity and Enantioselectivity of Lipases in Organic Solvents 67 Francesco Secundo Introduction 67 Lipase Formulations and their Activity and Enantioselectivity in Neat Organic Solvent 68 Why do Additives Affect the Activity and Enantioselectivity of Lipases in Organic Solvent? 73 Conclusions 76 References 76

Contents

6

6.1 6.2 6.3 6.4 6.5 6.6 6.7

7 7.1 7.1.1 7.1.2 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.2.7 7.3 7.3.1 7.3.2 7.3.3 7.4

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7

Direct Esterification with Dry Mycelia of Molds: a (Stereo)selective, Mild and Efficient Method for Obtaining Structurally Diverse Esters 79 Francesco Molinari, Diego Romano, Raffaella Gandolfi, Lucia Gardossi, Ulf Hanefeld, Attilio Converti and Patrizia Spizzo Mycelia and Biotransformations in Organic Media 79 Screening and Microbiological Aspects 79 Production of Acetate 81 Stereoselective Esterifications of Racemic Alcohols 83 Stereoselective Esterifications of Racemic Carboxylic Acids 85 Partition Phenomena and Equilibrium of Esterification Reactions 88 Conclusions 91 References 91 Factors Affecting Enantioselectivity: Allosteric Effects 93 Elisabeth Egholm Jacobsen and Thorleif Anthonsen How to Provide Enantiopure Compounds 93 Kinetic Resolution of Racemic Mixtures Catalyzed by Enzymes 94 Absolute Configurations in Resolution 95 Factors Affecting the Enantiomeric Ratio E 96 Is the E-value Really Constant? 96 Influence of the Reaction Medium on the E-value 96 Influence of Enzyme Immobilization on the E-value 97 Enzyme Inhibition 97 Enantioselective Inhibition and Activation: Allosteric Effects 97 The E-value of CALB is Influenced by R-Alcohols 99 Is a Changing E Caused by the Slow or the Fast Enantiomer? 102 Asymmetrization of Prochiral Compounds 103 Asymmetrization of Prochiral Dicarboxylates: Single-Step Process 103 Asymmetrization of Prochiral Diol: Double-Step Process 105 Is the e.e. Constant During Asymmetrization Reactions? 105 Conclusions 106 References 107 Kinetic Resolution of Sec-alcohol in Non-conventional Media 109 Maja Habulin, Mateja Primožicˇ and Željko Knez Introduction 109 SCFs – Replacement for Organic Solvents in Biocatalysis 111 Effect of Pressure 112 Effect of the Acyl Donor/Alcohol Molar Ratio 114 ILs – Solvents for Sustainable Technology in Biocatalysis 114 ILs, Based on the N, N’-Dialkylimidazolium Cations as Reaction Media 116 ILs/SCFs Biphasic Systems as Promising Media for Biocatalysis 117

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Contents

8.8 8.9 8.10

The [bmim][PF6]/SC-CO2 System as a Reaction Medium 117 Effect of Acyl Donor Concentration 119 Conclusion 120 References 120

9

Strategies for the Biocatalytic Lipophilization of Phenolic Antioxidants 123 Maria H. Katsoura, Eleni Theodosiou, Haralambos Stamatis and Fragiskos N. Kolisis Introduction 123 Materials and Methods 125 Materials 125 Enzymatic Acylation Procedure 125 Analytical Methods 125 Purification and Chemical Structure Determination of Esters 125 Results and Discussion 125 Modification of Natural Antioxidants in Organic Solvents 126 Enzymatic Acylation of Rutin and Silybin with Dicarboxylic Acids 126 Effect of Organic Solvent 127 Effect of Substrate Concentration 127 Modification of Natural Antioxidants in Ionic Liquid Media 128 Enzymatic Acylation of Natural Polyhydroxylated Compounds 128 Effect of Substrate Concentration 130 Effect of Acyl Donor Nature: Synthesis of Hybrid Antioxidants 130 Conclusions 131 References 132

9.1 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.3 9.3.1 9.3.1.1 9.3.1.2 9.3.1.3 9.3.2 9.3.2.1 9.3.2.2 9.3.2.3 9.4

10 10.1 10.2 10.3 10.4 10.5 10.6

11

11.1 11.2 11.3

Biocatalysis Applied to the Synthesis of Nucleoside Analogs 135 Vicente Gotor Introduction 135 Chemoenzymatic Modification of the Sugar 136 Resolution and Anomeric Separation 143 Biotransformations that Modify the Base 145 Transglycosylation for the Synthesis of Nucleosides 147 Summary 149 References 150 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus 153 Francisco J. Plou, Miguel Alcalde, Iraj Ghazi, Lucía Fernández-Arrojo and Antonio Ballesteros Introduction 153 Purification of Fructosyltransferase in Pectinex Ultra SP-L 155 Properties of Fructosyltransferase from A. aculeatus 157

Contents

11.3.1 11.3.2 11.3.3 11.3.4 11.3.5 11.4 11.4.1 11.4.2 11.4.3 11.5 11.5.1 11.5.2 11.6

12

12.1 12.2 12.3 12.4 12.5 12.6

13 13.1 13.2 13.2.1 13.2.2 13.2.3

Substrate Specificity 157 Effect of pH and Temperature 158 Influence of Chemicals 158 Kinetic Behavior 159 Fructooligosaccharide Production 159 Immobilization of Fructosyltransferase from A. aculeatus 161 Sepabeads EC-EP as Immobilization Carriers 161 Effect of pH and Ionic Strength on Immobilization 162 Application of Immobilized Biocatalysts to Fructooligosaccharide Synthesis 164 Fructooligosaccharide Production Using Sugar Beet Syrup and Molasses 164 Sugar Beet Syrup and Molasses as Low-cost Feedstock for Fructooligosaccharide Synthesis 164 Batch Production of Fructooligosaccharide 167 Conclusions 168 Acknowledgments 168 References 168 Hydantoin Racemase: the Key Enzyme for the Production of Optically Pure a-Amino Acids 173 Francisco Javier Las Heras-Vázquez, Josefa María Clemente-Jiménez, Sergio Martínez-Rodríguez and Felipe Rodríguez-Vico Introduction 173 Search for New Hydantoin Racemases and Molecular Characterization 175 Biochemical Characterization of Hydantoin Racemase Enzymes 180 Substrate Enantioselectivity and Kinetic Analysis of Hydantoin Racemases 181 Proposal for a Reaction Mechanism of Hydantoin Racemase Enzymes 183 Design of a Tailormade Recombinant Biocatalyst Including Hydantoin Racemase Enzymes for Optically Pure D-Amino Acid Production 187 Acknowledgments 192 References 192 Chemo-enzymatic Deracemization Methods 195 Davide Tessaro, Gianluca Molla, Loredano Pollegioni and Stefano Servi Introduction 195 Deracemization Methods for α- and β-Hydroxy Acids 196 Deracemization of Hydroxy Acids by DKR (Hydrolytic Enzymes + Ruthenium-based Racemization Catalysts) 197 Deracemization of Hydroxy Acids by DKR with a Two-enzyme System 198 Deracemization of Hydroxy Acids by Stereoinversion 199

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Contents

13.2.4 13.3 13.4 13.4.1 13.4.1.1 13.4.1.2 13.4.1.3

13.4.2 13.4.2.1 13.5 13.5.1 13.5.1.1 13.5.1.2 13.5.2 13.5.2.1 13.5.2.2 13.5.2.3 13.5.3 13.5.3.1 13.5.3.2 13.6

14

14.1 14.2 14.2.1 14.2.2 14.3 14.4 14.4.1 14.4.2 14.4.3 14.5

Deracemization of Hydroxy Acids by Microbial Stereoinversion 200 Deracemization of α-Hydroxy Nitriles 201 Deracemization of α-Amino Acids 202 Deracemization of α-Amino Acids by Stereoinversion 202 Deracemization by Stereoinversion via the Two-enzyme System D-Amino Acid Oxidase and L-Amino Transferase 202 Deracemization by Stereoinversion via the Two-enzyme System D-Amino Acid Oxidase and L-Leucine Dehydrogenase 204 Deracemization by Stereoinversion via the Three-enzyme System L-Amino Acid Oxidase, D-Amino Transferase and Amino Acid Racemase 204 Deracemization of α-Amino Acids via DKR 205 Deracemization of α-Amino Acids via Enzyme-catalyzed DKR Coupled with In Situ Racemization 205 Useful Enzymes for Deracemization Methods 213 Amino Acid Oxidases 213 D-Amino Acid Oxidase (EC 1.4.3.3) 213 L-Amino Acid Oxidase (EC 1.4.3.2) 216 Amino Acid Racemases 217 PLP-dependent Racemases 217 PLP-independent Racemases 220 Mandelate Racemase (EC 5.1.2.2) 221 Transaminases 221 L-Amino Transferases (EC 2.6.1.x) 222 D-Amino Transferases (EC 2.6.1.21) 223 Summary and Outlook 223 References 223 Nitrilases from Filamentous Fungi 229 Ludmila Martínková, Vojtech Vejvoda, Ondrej Kaplan, Vladimír Kren, Karel Bezouška and Maria Cantarella Introduction 229 Distribution and Evolutionary Relationship of Fungal Nitrilases 230 Molecular Genetic Analysis 230 Selection and Screening of Nitrilase Activity 232 Structural Properties 234 Catalytic Properties 236 Reaction Mechanism 236 Substrate Specificity 238 Activity and Stability 240 Conclusions and Outlook 242 Acknowledgment 243 References 243

Contents

15

15.1 15.2 15.2.1 15.2.2 15.3 15.3.1 15.3.2 15.3.3

16

16.1 16.2 16.3 16.4 16.5 16.6 16.7

17

17.1 17.2 17.3 17.4 17.5 17.6

Nitrilase- and Nitrile Hydratase-catalyzed Enantioselective Preparation of Non-proteinogenic Amino Acids 247 Norbert Klempier and Margit Winkler Introduction 247 Nitrile Hydratase/Amidase Biotransformations 249 Protecting Groups for Amino Nitriles 249 Enantioselective Hydrolysis of β-Amino Nitriles 250 Nitrilase Biotransformations 253 Enantioselective Hydrolysis of β-Amino Nitriles 253 Enantioselective Hydrolysis of γ-Amino Nitriles 255 Nitrile Hydratase Activity of Nitrilases 257 References 258 Nitrilases in the Enantioselective Synthesis of α-Hydroxycarboxylic Acids 261 Fred van Rantwijk, Cesar Mateo, Andrzej Chmura, Bruno C. M. Fernandes and Roger A. Sheldon Routes to Enantiomerically Pure α-Hydroxycarboxylic Acids 261 Nitrilase-mediated Hydrolysis of Cyanohydrins 262 A Bienzymatic Approach to Enantiopure 2-Hydroxycarboxylic Acids 264 Stabilization of NLases as Cross-linked Enzyme Aggregates 265 Hydrocyanation and Hydrolysis in a Bienzymatic Cascade 265 Nitrilases Acting as Nitrile Hydratases 267 Conclusion 270 Acknowledgments 271 References 271 UF-Membrane Bioreactors for Kinetics Characterization of Nitrile Hydratase–Amidase-catalyzed Reactions: a Short Survey 273 Maria Cantarella, Alberto Gallifuoco, Agata Spera, Laura Cantarella, Ondrˇej Kaplan and Ludmila Martínková Introduction 273 Experiment Design 275 Temperature Dependence of the Nitrile Hydratase–Amidase Cascade System 275 CSMR Investigations 277 Substrate Concentration Effects on the Reaction Rate, Enzyme Stability, Substrate Conversion, and Reactor Capacity 279 Concluding Remarks 283 Acknowledgments 285 References 285

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18

18.1 18.2 18.2.1 18.2.1.1 18.2.1.2 18.2.2 18.2.3 18.2.3.1 18.2.3.2 18.3 18.4

19 19.1 19.2 19.2.1 19.2.2 19.2.3 19.2.4 19.3 19.4

20 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8

Enzymes Catalyzing C−C Bond Formation for the Synthesis of Monosaccharide Analogs 287 Laurence Hecquet, Virgil Hélaine, Franck Charmantray and Marielle Lemaire Introduction 287 Recent Syntheses Involving Transketolase and Fructose-1,6bisphosphate Aldolase 287 DHAP Syntheses 288 DHAP Synthesis from Dihydroxyacetone 289 DHAP Synthesis from Rac-Glycidol 290 Synthesis of Aminocyclitols 291 Synthesis of 5-D-Xylulose and 5-D-Xylose Analogs 293 Synthesis of 5-halo-D-xylulose 293 Synthesis of 5-thio-D-xylopyranose 21 293 Modification of Substrate Specificity of Yeast Transketolase 295 Conclusion 296 References 297 Novel Strategies in Aldolase-catalyzed Synthesis of Iminosugars 299 Pere Clapés, Georg A. Sprenger and Jesús Joglar Introduction 299 DHAP-Aldolase-mediated Synthesis of Iminosugars from N-Cbzamino Aldehydes 301 Reaction Media 301 Aldolase-catalyzed Aldol Additions of DHAP to N-Cbz-Amino Aldehydes 301 Effect of N-Protecting Groups 304 Synthesis of Iminosugars: Reductive Amination 306 D-Fructose-6-Phosphate Aldolase as Catalyst for Iminosugar Synthesis 307 Summary and Outlook 309 References 309 Biocatalytic Asymmetric Oxidations with Oxygen 313 Roland Wohlgemuth Introduction 313 Biocatalytic Asymmetric Oxidations with Oxidases 317 Biocatalytic Asymmetric Oxidations with Peroxidases 319 Biocatalytic Asymmetric Oxidations with Dehydrogenases 320 Biocatalytic Asymmetric Oxidations with Monooxygenases 321 Biocatalytic Asymmetric Oxidations with Dioxygenases 325 Biocatalytic Asymmetric Oxidations with Other Enzymes 328 Outlook 331 Acknowledgments 331 References 332

Contents

21

21.1 21.2 21.3 21.4 21.4.1 21.4.2 21.4.3 21.4.4 21.5 21.6 21.6.1 21.6.1.1 21.6.1.2 21.6.2 21.6.3 21.7

Second Generation Baeyer–Villiger Biocatalysts 339 Veronique Alphand, Marco W. Fraaije, Marko D. Mihovilovic and Gianluca Ottolina Introduction 339 BVMO Enzyme Platform 341 Engineering of BVMOs 342 Baeyer–Villiger Biooxidation in Synthetic Chemistry 347 Chemoselectivity 347 Dynamic Kinetic Resolutions 350 Regio- and Stereoselectivity 351 Natural Product and Bioactive Compound Synthesis 354 BVMOs in Stereoselective Sulfoxidations 357 Towards a Technology Platform 358 Fermentation Up-Scaling 358 Whole Cells 358 Enzyme 361 Immobilization of BVMOs 361 Self-sufficient Fusion Protein BVMOs 361 Outlook 363 References 363 Index

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Preface Asymmetric compounds cover a steadily increasing market share, e.g. as fine chemicals, chiral intermediates or pharmaceutical ingredients. The worldwide commercial volume of single isomers of chiral drugs currently approaches US$200 billion. Chirality dictates how stereoisomers interact with biological systems. Because of the often much improved biological specificity profiles of pure enantiomers as compared to their racemates, methods for resolving racemates and for preparing single enantiomers are in high demand. For the challenge of asymmetric synthesis to generate complex chiral compounds in high enantiomeric purity and yield, biocatalysis offers a tremendous advantage due to the homochiral nature of protein catalysts, which offer unparalleled levels of stereoselectivity and reaction specificity, in addition to their remarkable catalytic efficiency under mild conditions and regenerative production from biological materials. Breakthrough technologies in modern molecular biotechnology research, development and application have recently paved the way for the rapid discovery and engineering of novel enzymes, and today allow the generation of biocatalysts that are optimally adaptable to even demanding industrial process parameters. Such features form the fundaments of so-called white biotechnology – the use of microorganisms and enzymes for industrial chemical production, which currently strongly benefits from political support because of its impact on sustainable development, lower energy consumption and independence from fossil raw materials. Rising oil prices from a shortage of crude oil resources and political uncertainty in oil-producing countries; global warming due to greenhouse gases; increasing population; and environmental pollution are strong drivers for this trend. Utilization of biocatalysis for chiral chemical synthesis and pharmaceutical manufacturing has come a long way, from highly specific niche applications a century ago, through hundreds of small to medium scale industrial processes in the fine chemical sector, e.g. including antibiotics, to even bulk processes for nonchiral commodity chemicals such as acrylamide. The conversion of traditional industrial processes to biotechnological alternatives is still at an early stage, because high costs in the development and adaptation of new processes have slowed down a substitution; development cycles for such biocatalytic processes used to be longer than anticipated and considerably longer than those for comparable chemical process alternatives. In the long run, however, biocatalytic processes have often Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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Preface

proven to be economically feasible, ecologically advantageous and more sustainable than current chemical technologies because of the intrinsic advantages of biocatalysts in higher reaction selectivity, milder reaction conditions and potential use of inexpensive regenerable resources. After all, the tools of the trade have changed dramatically over the past decade, and with current advanced technology in protein design and engineering, further developments are expected to strongly gain momentum for the immediate future. As one of the most impressive examples, the newly developed one-step fermentation process for vitamin B2 at BASF has cut CO2 emissions by 30%, production costs by 40%, consumption of resources by 60% and waste generation by 95% when compared to the conventional eightstep chemical synthesis. Still, the field of biocatalysis research for preparative synthesis poses a range of intellectual frontiers and needs further developments to broaden the range of applicable reaction types, addressable target structures and advanced method integration. There is both general and industrial interest in further research to extend the scope of biocatalysis for asymmetric synthesis, and a collaboration of organic chemists, biochemists, molecular geneticists and biotechnical engineers is needed for success. The development of biocatalytic methodologies undoubtedly requires strong interdisciplinary and transdisciplinary research cooperation, and meeting the challenges for an environmentally friendly, sustainable process design adds another dimension, from the supply and efficient use of raw materials to the minimization and recycling of enzymes, by-products and waste under economical constraints. This book summarizes the efforts and current state of the art in several important arenas of biocatalysis research that has been coordinated within the activities of the European Union-funded COST network D25, entitled ‘Applied Biocatalysis: Stereoselective and Environmentally Friendly Reactions Catalysed by Enzymes’, over the last five years. The topics of the chapters span from modern assay technologies for enzyme screening over different factors influencing enzyme selectivities, including the consequences from enzyme formulation and various solvent effects, to the manifold of preparative applications. More than half of the book chapters deal with the various conceptual strategies and synthetic opportunities available for the rational synthesis or targeted modification of different compound classes, such as phenolic natural products, nucleoside analogs, monosaccharides and oligosaccharides, iminosugars, proteinogenic and non-proteinogenic amino acids, nitriles, hydroxy acids, and various oxidation products, including lactones from enzymatic Baeyer–Villiger reactions. We are grateful to all those friends and colleagues who helped to start, and then participated in, the D25 action and who kindly contributed to this project as authors of dedicated and informative chapters in order to share their expertise with you. It is our hope that this volume will encourage scientific discussion and foster imaginative new developments in applied biocatalysis for tomorrow’s novel applications. Wolf-Dieter Fessner and Thorleif Anthonsen Darmstadt and Trondheim, October 2008

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List of Contributors Miguel Alcalde Departamento de Biocatálisis Instituto de Catálisis y Petroleoquímica CSIC Cantoblanco 28049 Madrid Spain Veronique Alphand Université Paul Cézanne Biosciences – FRE CNRS 3005 Case 432 Av. Escadrille Normandie-Niemen 13397 Marseille cedex 20 France Thorleif Anthonsen Norwegian University of Science and Technology Department of Chemistry 7491 Trondheim Norway Antonio Ballesteros Departamento de Biocatálisis Instituto de Catálisis y Petroleoquímica CSIC Cantoblanco 28049 Madrid Spain

Karel Bezouška Charles University Prague Faculty of Science Department of Biochemistry 128 40 Prague Czech Republic Laura Cantarella Department of Industrial Engineering University of Cassino Via di Biasio 43 03043 Cassino (FR) Italy Maria Cantarella University of L’Aquila Department of Chemistry Chemical Engineering and Materials 67040 Monteluco di Roio – L’Aquila Italy Franck Charmantray Université Blaise Pascal CNRS UMR 6504 – SEESIB 63177 Aubière Cedex France


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List of Contributors

Andrzej Chmura Delft University of Technology Department of Biotechnology Laboratory of Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands Pere Clapés Catalonia Institute for Advonced Chemistry (IQAC)-CSIC. Group of Biotransformation and Bioactive Molecules Jordi Girona 18-26 08034 Barcelona Spain Josefa María Clemente-Jiménez Departamento de Química-Física Bioquímica y Química Inorgánica Edificio CITE I Carretera de Sacramento S/N La Cañada de San Urbano 04120 Almería Spain Attilio Converti University of Genoa Department of Chemical and Process Engineering ‘G.B. Bonino’ Via Opera Pia 15 16145 Genoa Italy Bruno C. M. Fernandes Delft University of Technology Department of Biotechnology Laboratory of Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands

Lucía Fernández-Arrojo Departamento de Biocatálisis Instituto de Catálisis y Petroleoquímica CSIC Cantoblanco 28049 Madrid Spain Marco W. Fraaije University of Groningen Groningen Biomolecular Sciences and Biotechnology Institute Biochemical Laboratory Nijenborgh 4 9747 AG Groningen The Netherlands Alberto Gallifuoco University of L’Aquila Department of Chemistry Chemical Engineering and Materials 67040 L’Aquila Italy Raffaella Gandolfi University of Milan Institute of Organic Chemistry ‘Alessandro Marchesini’ Via Venezian 21 20133 Milan Italy Lucia Gardossi Università degli Studi di Trieste Dipartimento di Scienze Farmaceutiche Laboratory of Applied and Computational Biocatalysis Piazzale Europa 1 34127 Trieste Italia


Iraj Ghazi Departamento de Biocatálisis Instituto de Catálisis y Petroleoquímica CSIC Cantoblanco 28049 Madrid Spain Vicente Gotor Universidad de Oviedo Departamento de Química Orgánica e Inorgánica Instituto de Biotecnología de Asturias 33006 Oviedo Spain Maja Habulin University of Maribor Faculty of Chemistry and Chemical Engineering Laboratory for Separation Processes and Product Design Smetanova 17 2000 Maribor Slovenia Ulf Hanefeld Delft University of Technology Department of Biotechnology Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands Laurence Hecquet Université Blaise Pascal Synthèse et Etudes de Systèmes à Intérêt Biologique UMR 6504 24 avenue des Landais 63177 Aubiere Cedex France

Virgil Hélaine Université Blaise Pascal CNRS UMR 6504 – SEESIB 63177 Aubière Cedex France Francisco Javier Las Heras-Vázquez Departamento de Química-Física Bioquímica y Química Inorgánica Edificio CITE I Carretera de Sacramento S/N La Cañada de San Urbano 04120 Almería Spain Elisabeth Egholm Jacobsen Norwegian University of Science and Technology Department of Chemistry 7491 Trondheim Norway Jesús Joglar Catalonia Institute for Advonced Chemistry (IQAC)-CSIC. Group of Biotransformation and Bioactive Molecules Jordi Girona 18-26 08034 Barcelona Spain Ondrej Kaplan Academy of Sciences of the Czech Republic Institute of Microbiology Center of Biocatalysis and Biotransformation 142 20 Prague Czech Republic

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Maria H. Katsoura University of Ioannina Department of Biological Applications and Technologies Laboratory of Biotechnology 45110 Ioannina Greece Norbert Klempier Technische Universität Graz Institut für Organische Chemie Stremayrgasse 16 8010 Graz Austria Željko Knez University of Maribor Faculty of Chemistry and Chemical Engineering Laboratory for Separation Processes and Product Design Smetanova 17 2000 Maribor Slovenia Fragiskos N. Kolisis National Technical University of Athens Chemical Engineering Department Biotechnology Laboratory 5 Iroon Polytechniou Str. Zografou Campus 15700 Athens Greece Vladimír Kren Academy of Sciences of the Czech Republic Institute of Microbiology Center of Biocatalysis and Biotransformation 142 20 Prague Czech Republic

Morten Kristensen Arinco Arla Foods Amba Mælkevejen 4 6920 Videbæk Denmark Marielle Lemaire Université Blaise Pascal CNRS UMR 6504 – SEESIB 63177 Aubière Cedex France Sergio Martínez-Rodríguez Departamento de Química-Física Bioquímica y Química Inorgánica Edificio CITE I Carretera de Sacramento S/N La Cañada de San Urbano 04120 Almería Spain Ludmila Martínková Academy of Sciences of the Czech Republic Institute of Microbiology Center of Biocatalysis and Biotransformation 142 20 Prague Czech Republic Cesar Mateo Delft University of Technology Department of Biotechnology Laboratory of Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands


Marko D. Mihovilovic Vienna University of Technology Institute for Applied Synthetic Chemistry Getreidemarkt 9 1060 Vienna Austria Francesco Molinari University of Milan Department of Food Science and Microbiology Via Celoria 2 20133 Milan Italy Gianluca Molla Università degli Studi dell’Insubria Dipartimento di Biotecnologie e Scienze Molecolari Via J.H. Dunant 3 21100 Varese Italy Bernd Nidetzky Graz University of Technology Institute of Biotechnology and Biochemical Engineering Petersgasse 12 8010 Graz Austria Gianluca Ottolina Istituto di Chimica del Riconoscimento Molecolare CNR Via Mario Bianco 9 20131 Milano Italy

Lars Haastrup Pedersen Aalborg University Department of Biotechnology Chemistry and Environmental Engineering Sohngårdsholmvej 49 9000 Aalborg Denmark Francisco J. Plou Departamento de Biocatálisis Instituto de Catálisis y Petroleoquímica CSIC Cantoblanco 28049 Madrid Spain Loredano Pollegioni Università degli Studi dell’Insubria Dipartimento di Biotecnologie e Scienze Molecolari Via J.H. Dunant 3 21100 Varese Italy Mateja Primožicˇ University of Maribor Faculty of Chemistry and Chemical Engineering Laboratory for Separation Processes and Product Design Smetanova 17 2000 Maribor Slovenia Fred van Rantwijk Delft University of Technology Department of Biotechnology Laboratory of Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands

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Jean-Louis Reymond University of Berne Department of Chemistry and Biochemistry Freiestraße 3 3012 Berne Switzerland Sinthuwat Ritthitham Aalborg University Department of Biotechnology Chemistry and Environmental Engineering Sohngårdsholmvej 49 9000 Aalborg Denmark Felipe Rodríguez-Vico Departamento de Química-Física Bioquímica y Química Inorgánica Edificio CITE I Carretera de Sacramento S/N La Cañada de San Urbano 04120 Almería Spain Diego Romano University of Milan Department of Food Science and Microbiology Via Celoria 2 20133 Milan Italy Francesco Secundo Istituto di Chimica del Riconoscimento Molecolare CNR Via Mario Bianco 9 20131 Milano Italy

Stefano Servi Politecnico di Milano Dipartimento CMIC ‘G. Natta’ Via Mancinelli 7 20131 Milano Italy Roger A. Sheldon Delft University of Technology Department of Biotechnology Laboratory of Biocatalysis and Organic Chemistry Julianalaan 136 2628 BL Delft The Netherlands Agata Spera Department of Chemistry Chemical Engineering and Materials University of L’Aquila 67040 L’Aquila Italy Patrizia Spizzo Università degli Studi di Trieste Dipartimento di Scienze Farmaceutiche Laboratory of Applied and Computational Biocatalysis Piazzale Europa 1 34127 Trieste Italia Georg A. Sprenger Universität Stuttgart Institute of Microbiology Allmandring 31 70569 Stuttgart Germany Haralambos Stamatis University of Ioannina Department of Biological Applications and Technologies Laboratory of Biotechnology 45110 Ioannina Greece


Davide Tessaro Politecnico di Milano Dipartimento CMIC ‘G. Natta’ Via Mancinelli 7 20131 Milano Italy Eleni Theodosiou National Technical University of Athens Chemical Engineering Department Biotechnology Laboratory 5 Iroon Polytechniou Str. Zografou Campus 15700 Athens Greece Malene S. Thomsen Research Centre Applied Biocatalysis Petersgasse 14 8010 Graz Austria.

Vojtech Vejvoda Academy of Sciences of the Czech Republic Institute of Microbiology Center of Biocatalysis and Biotransformation 142 20 Prague Czech Republic Margit Winkler Technische Universität Graz Institut für Organische Chemie Stremayrgasse 16 8010 Graz Austria Roland Wohlgemuth Sigma-Aldrich Research Specialities Industriestrasse 25 9470 Buchs Switzerland

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1 Fluorescence Assays for Biotransformations Jean-Louis Reymond

1.1 Introduction

High-throughput activity screening in microbial collections, expression libraries of enzyme mutants, or metagenomic libraries using enzyme activity assays is one of the key steps in the development of new or improved biocatalysts. Ideally, screening is carried out with the authentic reaction of interest under its operating conditions and reaction progress is monitored using analytical instruments such as high-performance liquid chromatography, gas chromatography, nuclear magnetic resonance, or mass spectrometry, or simply thin-layer chromatography. In most laboratories, however, such analyses are not realizable in high-throughput or in the format imposed by the expression conditions of the library under study. In such cases the enzyme discovery process can be assisted by a fluorescence assay. Fluorescence can be recorded with high sensitivity in a variety of settings, in particular in a high-throughput screening format such as microtiter plates, micro-arrays, or fluorescence-activated cell sorting. Fluorescence is also usually directly visible by the eye under illumination with an ultraviolet (UV) lamp. Fluorescence monitoring of a biotransformation is possible either by coupling the reaction of interest to a fluorogenic sensor system, for example an enzymecoupled assay linking the reaction to the production or consumption of NAD+ or by using a fluorogenic model substrate instead of the authentic substrate. Fluorogenic substrates are particularly useful for high-throughput screening and routine identification of an enzyme during cloning when it has been established that this enzyme catalyzes the desired reaction. In recent years the author and others have reported on a variety of fluorescence sensors and substrates that expand the chemistry of classical assays to a broad diversity of reactions and substrates. The present chapter reviews the development of fluorogenic substrates based on umbelliferone and related fluorescent phenols by the author’s research group and related reports by others. A more general presentation of recent advances in high-throughput screening assays for biotransformations has been recently published [1]. Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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1 Fluorescence Assays for Biotransformations

1.2 Alcohol Dehydrogenases (ADHs) and Aldolases

Alcohol dehydrogenases and aldolases are some of the most synthetically significant enzymes in biocatalysis because they carry out classical and highly useful reactions with ketones and aldehydes that generate new asymmetric centers. Although only a limited number of these enzymes are currently in use, their potential for complex asymmetric synthesis is very high and efficient fluorogenic substrates are particularly important for facilitating research in this area. 1.2.1 Chiral Fluorogenic ADH Substrates

Alcohol dehydrogenases can be assayed by following the conversion of NAD(P)+ to NAD(P)H at 340 nm, either by UV or by fluorescence. However, the signal is often obscured by contaminants or other assay components if the assay is done in whole cells or in the presence of other chromophores and the reduced forms of the co-factors are not very stable. Signal stability is also a problem in indirect assays where the reduced co-factors are detected in a colorimetric secondary reaction such as the formazane test [2]. As an alternative to these methods, one can consider using a fluorogenic or chromogenic alcohol substrate as a reporter for the reaction. Umbelliferyl ethers of chiral 1,3-diols such as 1, 2, or 3 that serve as fluorogenic substrates for alcohol dehydrogenases have been developed (Scheme 1.1) [3–5]. The alcohols are obtained by alkylation of an umbelliferyl anion with activated precursors and this allows various substitution patterns and stereochemistries to be installed around the enzyme reactive functional group. The enzyme

Scheme 1.1 Chiral fluorogenic alcohol dehydrogenase substrates and assay conditions.

1.2 Alcohol Dehydrogenases (ADHs) and Aldolases

to the corresponding carbonyl (e.g. 4) oxidizes the alcohol function and this spontaneously liberates the fluorescent product umbelliferone 5 by β-elimination. This secondary reaction occurs above pH 7 and can be catalyzed by the addition of bovine serum albumin (BSA). The alkaline pH is also necessary because the fluorescent umbelliferone anion only exists above pH 7. The blue fluorescent signal produced around 440 nm increases over 20-fold upon conversion of the alcohol substrates to umbelliferone and is visible by the naked eye under illumination with UV light, which is particularly convenient during screening. On the other hand the β-elimination process has a half-life of 3–15 min depending on the substrate, which limits the reaction kinetics if the alcohol oxidation itself is very fast. Therefore the assay is only suitable for kinetic studies under slow reaction conditions, for example using low enzyme concentrations. 1.2.2 Fluorogenic Aldolase Probes

The discovery of aldolase catalytic antibodies in the 1990s [6–8] led to an interest in screening biocatalytic aldol reactions of non-natural aldolase-type substrates. The ADH assay above can be modified in the form of aldol substrates such as 6 to assay retro-aldolization reactions via a retro-aldol/β-elimination sequence. The aldolase substrate 6 and analogs are prepared by direct synthesis from aldehyde 4 (Scheme 1.2) [9, 10]. Aldehyde 4 is unstable towards β-elimination in water, but can be handled in organic solvent. For instance, the reaction of aldehyde with silyl enolethers under catalysis by SnCl4 at low temperature in dichloromethane proceeds in excellent yields without β-elimination to deliver a stereoisomeric mixture of aldol 6, which can then be separated by high-performance liquid chromatography for individual assays. The retro-aldol reaction can also be assayed with aldol 7, which follows the same mechanism as 6, as well as with aldols 8 and 9, which directly yield a fluorescent aldehyde product upon retro-aldolization, as shown by List et al. [11]. The advantage of assaying retro-aldol reactions as opposed to the

Scheme 1.2 Fluorogenic substrates for aldolase catalytic antibodies and related biocatalysts.

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Scheme 1.3 Fluorogenic substrates for transaldolase and transketolase.

forward reaction lies in the possibility of testing individual stereoisomers of the aldol product for reactivity, leading to a prediction of stereoselectivity, as was analyzed using all eight diastereoisomers of aldol 6 [10]. 1.2.3 Transaldolases and Transketolases

Related systems were later developed for transaldolases 10–12 (Scheme 1.3) [12]. The fructo/tagato stereoselectivity of various transaldolases was determined by fluorescence for the stereoisomeric substrate pair 11/12. However, the reactivity of the substrates towards transaldolases is much lower than with the natural substrate due to the replacement of the phosphate group at position 6 of the natural fructose-6-phosphate substrate with the neutral, aromatic coumarin ether, which is not well recognized by the enzyme. Sevestre et al. [13] reported substrate 13 as a fluorogenic substrate for transketolases, based on a similar fluorescence release mechanism. It should be mentioned that most natural aldolase enzymes can also be assayed using enzyme-coupled systems relaying the reaction to a redox process with NAD+. The formation of NADH by active microbial colonies in expression libraries of mutant enzymes was detected colorimetrically in agar plates using phenazine methosulfate and nitroblue tetrazolium, which forms an insoluble precipitate. The assay was used by Williams et al. [14] and Woodhall et al. [15] for evolving sialic acid aldolases to accept non-natural aldehyde acceptors. 1.2.4 Enolase Probe

Aldolase-type biocatalysts can generally also be expected to catalyze the enolization of the ketone donor. However, directly detecting enolization by fluorescence is not possible. It was recently found that dihydroxyacetone coumarin ether 14 functions

1.3 Lipases and Esterases

Scheme 1.4 Substrates to assay enolization and deprotonation at carbon.

as a fluorogenic enolization probe in aqueous buffer (Scheme 1.4) [16]. Enol formation is followed by β-elimination of umbelliferone and the corresponding fluorescence signal. The probe detects the activity of a variety of small molecule enolization catalysts and is generally useful for screening aldolase type biocatalysts [17]. Alkylation of umbelliferone with allyl bromide, dihydroxylation of the allyl ether double bond, silylation of the primary alcohol, oxidation of the secondary alcohol to the ketone, and acidic deprotection provide excellent yields of the probe. Enolization involves a deprotonation step at carbon, which is a fundamental reaction step of interest in studying the principles of enzyme action. Investigations of biocatalytic carbon deprotonation have focused on the chromogenic ring opening of nitrobenzoxazole 15 to form the yellow product 2-cyano-4-nitrophenol 16. The reaction is promoted by weak bases such as carboxylate side chains in a hydrophobic environment, in particular with albumins and catalytic antibodies [18–21]. The unstable aldehyde 4 (Scheme 1.1) and related carbonyls also provide interesting fluorogenic substrates for such reactions and the β-elimination process is indeed catalyzed by BSA in an enzyme-like manner [3].


Lipases and esterases can be assayed using pH indicators. A particularly elegant implementation of this principle was reported by the group of Kazlauskas for screening enantioselectivities using the Quick E assay [22, 23]. A variety of other high-throughput screening assays have been reported based on the chemoselective detection of the reaction products from untagged esters. These include fluorogenic hydrazone formation with acetaldehyde produced by lipase/esterase-catalyzed transesterification of vinyl esters in organic solvents [24], the detection of acetic acid produced from acetate esters using an enzyme kit [25], or the back titration of periodate with adrenaline after oxidation of the glycerol release from triglyceride hydrolysis [26, 27]. Various chromogenic and fluorogenic esters for lipases and esterase are also available commercially in the form of simple esters of umbelliferone or

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p-nitrophenol. Unfortunately such substrates are often relatively unreactive towards the enzyme while displaying a high level of non-specific hydrolysis in the reaction medium. In order to overcome these limitations various assays and substrates that significantly improve the fluorogenic detection of lipases and esterases have now been developed. 1.3.1 Assays on Solid Support

The disadvantage of esters of umbelliferone or nitrophenol esters as lipase substrates lies in their low aqueous solubility and high non-specific reactivity in water. Because lipases are interfacial enzymes, there has been a search for an assay format where these substrates would be presented to the enzyme solution in stable, insoluble form over a high surface material. The best implementation of this principle consists of impregnating silica gel plates (used for analytical thinlayer chromatography) with a solution of the umbelliferone esters in dichloromethane [28]. Evaporation of the solvent leaves the substrate homogeneously adsorbed on the surface. The silica gel plates can then be used for assaying enzymes by simply adding a drop of test solution to the surface. The buffer impregnates the plate and the substrates are dissolved and hydrolyzed if an active enzyme in present. This format is extremely practical since it allows the assay volume to be reduced to only 1 µl per assay. Its use has been demonstrated in high-throughput by profiling 35 different enzymes across 20 different ester substrates, realizing over 7000 individual assays in a matter of hours. The cyclohexane carboxylic ester 17, which releases 4-methylumbelliferone 18 as a fluorescent product, appeared as the most uniformly reactive substrate across all lipases and esterases tested (Scheme 1.5). It should be mentioned that silica gel plates can be impregnated with fluorogenic substrates for other enzymes, for example umbelliferyl glycosides for testing glycosidases. The photograph insert shows a typical plate impregnated with substrate 17 under UV illumination after reaction of 1-µl reactions containing various lipases (columns 1–10) or controls (BSA) (columns 11 and 12) under various conditions (lanes A–L) (see the original publication for details). Substrate micro-arrays for testing lipases have also been investigated (Scheme 1.6) [29]. The assay is based on an indirect tagging strategy involving periodate cleavage of a primary 1,2-diol product followed by chemoselective tagging with rhodamine sulfohydrazide. The tag becomes covalently attached to the surface whenever an aldehyde product has been formed and its red fluorescence can be quantified using a standard micro-array scanner. The 1,2-diol esters showed the desired chemical stability on the micro-array and the 1,2-diol could be successfully tagged chemoselectively using the hydrazide reagent without non-specific staining under appropriate washing conditions. However, most lipases proved unreactive in the assay and the most reactive enzymes on the micro-array were not those most active in solution, highlighting strong differences between the two reaction formats.


Scheme 1.5 Fluorescence lipase assay on a solid support.

Scheme 1.6 A substrate micro-array for testing lipases.

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1.3.2 The Clips-O Substrates with Periodate

The Clips-O substrates form one of the most reliable classes of fluorogenic and chromogenic substrates for hydrolytic enzymes, in particular for lipases (e.g. 19) [30–32]. These substrates are based on a double indirect release mechanism for the fluorescence signal exploiting sequential cleavage of a primary 1,2-diol product (e.g. 20) with sodium periodate to form an unstable aldehyde or ketone (e.g. 21), which undergoes β-elimination to release umbelliferone 5 or nitrophenol (Scheme 1.7). Sodium periodate oxidizes 1,2-diols and 1,2-aminoalcohols selectively, but does not interact with most of the other functional groups, in particular the substrates of the assay and the enzymes. This indirect release strategy separates the enzyme reactive functional group from the fluorescent label. In addition, the functional group reacting with the enzyme is non-activated and therefore much less prone to uncatalyzed spontaneous hydrolysis in the assay medium. The ClipsO substrate 19 is particularly reactive with lipases because the enzyme reactive ester is part of a 1,2-diol monoester function resembling the natural glyceride ester substrates of these enzymes [33]. Interestingly, it has been found that 19 not only reacts well with lipases, but also shows a high reactivity with esterases, such that the substrate serves as a general

Scheme 1.7 Clips-O substrates for lipases and esterases.


reference fluorogenic substrate for esterases and lipases. An enzyme fingerprinting study involving eight enantiomeric pairs of analogs of 19 with various acyl chain length showed that lipases and esterases can be distinguished by their chain length selectivity [34]. The corresponding nitrophenyl and dinitrophenyl derivatives 22 and 23 display similar reactivities [35]. As for the ADH and aldolase assays discussed above (Scheme 1.1), the secondary decomposition of the 1,2-diol product leading to umbelliferone may be kinetically limiting in the assay, implying that the substrates are not suitable for following the kinetics of very fast enzymes. 1.3.3 Esters of Fluorogenic Cyanohydrins and Hydroxyketones

Using aliphatic alcohols rather than acidic phenols in the ester function of lipase and esterase substrates is the key to obtaining selective probes for these enzymes. In the examples above the problem was solved by using an oxidative decomposition of the primary aliphatic alcohol product. Fluorogenic cyanohydrins and hydroxyketones that spontaneously react to form a fluorescent phenol following the β-elimination sequence presented above have been investigated as alternative reagent-free versions of fluorogenic aliphatic alcohols (Scheme 1.8) [36]. Reaction of the unstable aldehyde 21 with cyanotrimethyl silane (TMSCN) in dichloromethane and acidic hydrolysis of the trimethylsilyl (TMS) group affords cyanohydrin 25 without β-elimination. The cyanohydrin 25 can then be esterified with various acyl chlorides to afford the corresponding esters such as 24. These substrates undergo a fluorogenic reaction with lipases and esterases. The enzyme-catalyzed

Scheme 1.8 Fluorogenic cyanohydrins and hydroxyketone esters.

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ester hydrolysis liberates the cyanohydrin 25, which then undergoes a rapid and spontaneous decomposition in aqueous buffer to form umbelliferone as a fluorescent product. Interestingly, the equilibration of the cyanohydrin 25 to the aldehyde 21 is faster than the β-elimination. The fluorogenic hydroxyketone 14 discussed above (Scheme 1.4) can also be used to form esters such as 26 which can be used as fluorogenic substrates for lipases [37]. In this case, however, the esters are quite unstable despite being aliphatic alcohol esters. The relatively rapid spontaneous hydrolysis in this case is probably due to an assisted mechanism involving acyl transfer in the hydrated form of the ketone. 1.3.4 Fluorogenic Acyloxymethyl Ethers

Alkylation of umbelliferone with chloromethyl esters of various aliphatic acids affords the corresponding acyloxymethyl ethers (e.g. 27) in good yields [38]. These substrates undergo lipase/esterase-catalyzed hydrolysis in buffer to form umbelliferone (Scheme 1.9) [39]. The mechanism probably involves an unstable hemiacetal intermediate, which spontaneously reacts to formaldehyde and umbelliferone 5. The substrates can also be prepared in a two-step sequence involving alkylation of umbelliferone with 2-chloroketones followed by Baeyer–Villiger oxidation. The sequence is particularly interesting for obtaining the fluorogenic lactones 28 and 29, which only react with esterases [40]. The alkylation chemistry described above can also be applied with fluorescein to yield the corresponding fluorescein mono-ethers such as 30 as green fluorescent probes for lipases [41]. These substrates are water soluble due to the anionic carboxylate on the fluorescein group and can be used in pure aqueous buffer without any co-solvent. This latter class of substrates reacts extremely fast and specifically with the enzymes, with assay times under 1 min. An activity fingerprinting study

Scheme 1.9 Fluorogenic acyloxymethyl ethers for lipases and esterases.

1.4 Other Hydrolases

using various fluorescein mono-ether substrates and enzymes either in pure buffer or in buffer containing 20% v/v dimethylsulfoxide co-solvent showed that enzymes classified as esterases react faster in pure aqueous buffer, while those classified as lipases show a stronger reactivity in the presence of the co-solvent, which might be related to the fact that lipases are activated at interfaces. 1.3.5 FRET-Lipase Probes

FRET (fluorescence or Förster resonance energy transfer) is one of the most general principles for designing fluorogenic substrates for bond-cleaving reactions and is well known for protease and lipase substrates. Despite the strong influence of the double labels on reactivity, the use of FRET substrates for lipases has been investigated. A survey of various variations around the 1,2-diol monoester function led to the discovery that most lipases react strongly with the pyrene carboxylic ester 31, which is non-fluorescent due to intramolecular fluorescence quenching by the dinitrophenylamino group, to liberate the fluorescent pyrene butyrate product 32 [42]. The assay is carried out at a low substrate concentration due to the limited solubility and very low KM values for this substrate. Most interestingly, the substrate is extremely resistant towards non-specific hydrolysis, in particular at alkaline pH. This allows one to screen enzymes under strongly basic conditions, as illustrated for the Roche lipase L8 (Scheme 1.10). A survey of various lipases and esterases at pH 11 showed that most enzymes in fact lose their activity under these conditions.


Most of the reactivity principles used for fluorogenic lipase substrates are also applicable for other types of hydrolases. Such developments in the case of the epoxide hydrolases, amidases, esterases, and phosphatases have been investigated. Glycosidases were not investigated because the well-known aryl glycosides function perfectly well for their corresponding enzymes, with negligible non-specific reactivity in the absence of enzyme. On the other hand, there is still a need for developing fluorogenic substrates for nitrilases, a further important class of enzymes for biocatalysis. 1.4.1 Epoxide Hydrolases

The 1,2-diol product 20 in the Clips-O lipase substrate 19 (Scheme 1.7) can also form upon hydrolysis of the parent epoxide 33, leading to a convenient periodate coupled assay for epoxide hydrolases with a variety of epoxides (Scheme 1.11) [5, 32, 43]. These substrates are readily prepared in racemic form from umbelliferone by alkylation

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12


Scheme 1.10 FRET lipase substrate for alkaline assay of lipases.

with an allyl halide and epoxidation of the allyl ether with a peracid. The optically pure fluorogenic epoxides are obtained by asymmetric Sharpless dihydroxylation of the allyl ether, followed by regioselective ring closure. Such enantiomerically pure epoxides may be used for screening enantioselective epoxide hydrolases, although the problem of enantioselectivity is more complex. Indeed epoxide hydrolases enzymes may catalyze epoxide opening at both sides of the epoxide, leading to enantiomeric products [44]. Alternatively, the enzyme reaction may also lead to an enantioconvergent hydrolysis where a racemic epoxide is converted to a single enantiomer of the 1,2-diol [45]. The Clips-O epoxide 33–41 and related substrates are currently the only available selective fluorogenic substrates for EH. On the other hand various indirect assays have been reported to detect either the unreacted epoxide [46] or the carbonyl product resulting from periodate cleavage of the 1,2-diol product [26, 47, 48]. These assays are suitable for fluorescence or colorimetric assays for the hydrolysis of any epoxide of interest.


Scheme 1.11 Clips-O fluorogenic substrates for epoxide hydrolases.

1.4.2 Amidases and Proteases

1,2-Aminoalcohols are oxidized by sodium periodate to form the corresponding carbonyl products. The reaction is much faster than for 1,2-diols in a surprisingly pH-independent manner. The reaction can be used for designing non-volatile precursors of fragrant ketones and aldehydes by cyanide addition to form the cyanohydrins and reduction to the corresponding amino alcohol [49]. The oxidative cleavage of 1,2-aminoalcohol for designing selective substrates for amidases such as the phenacetylamide 42 suitable for screening penicillin G amidase has been exploited (Scheme 1.12) [32, 43]. A related fluorogenic substrate for ceramidase was also recently reported by others [50]. The phenacetyl derivative 43 also serves a fluorogenic substrate for amidases [37]. This substrate is however completely unstable towards alkaline conditions, probably due to hydrolysis by an assisted mechanism with an intramolecular acyl shift to the hydrated diol as discussed above for the corresponding lipase substrate. A chromogenic threonine derivative to form a selective HIV-protease substrate 44 has also been prepared [51]. A particularly elegant solution for the assay of proteases and acylases is offered by the fluorogenic detection of free amino acids by decomplexation of copper from calcein, which removes its quenching effect. This principle has been used for assays of acylases, amidases, and proteases (Scheme 1.13) [52, 53]. For the case of proteases combinatorial assays are particularly in demand for testing multiple peptides in parallel and determining the cleavage specificity [54]. New solutions

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Scheme 1.12 Clips-O acylase and amidase substrates.

Scheme 1.13 Copper calcein as a fluorogenic sensor for amino acids.

to this problem using cocktails of fluorescence-labeled peptides [55] or combinatorial peptide libraries were recently reported [56, 57]. 1.4.3 Phosphatases

Although phosphatases are only of moderate interest in organic synthesis, phosphatase enzymes and particularly phytases that hydrolyze phytic acids bearing multiple phosphate groups are of biotechnological interest. The standard substrates for phosphatases are nitrophenyl phosphates and related monophosphates of aromatic phenols. Di- and triphosphates 45 and 46 related to the Clips-O principle discussed above have been investigated (Scheme 1.14) [58]. The substrates

1.6 Conclusion

Scheme 1.14 Clips-O substrates for phosphatases.

are obtained from the diols by phosphorylation with dibenzyl phosphoramidite, followed by oxidation to the phosphate and debenzylation by hydrogenation. These phosphates display the expected reactivity towards phosphatases, however without selectivity for phytases against alkaline phosphatases, which can be differentiated rather on the basis of their pH rate profile.

1.5 Baeyer–Villigerases

Baeyer–Villiger monoxygenases (BVMOs) have been recently recognized as preparatively useful enzymes, in particular in relation to their enantioselectivity with prochiral ketones. It has recently been shown that 2-aryloxyketones 47–50 act as fluorogenic substrates for these enzymes (Scheme 1.15) [40]. The natural selectivity of oxidative cleavage leads to insertion of the oxygen atom on the side of the aryl ether to form an unstable lactone or acetal ester, which is rapidly hydrolyzed to form umbelliferone. The assay is useful for the detection of BVMOs in whole cells. However, in some cases oxidative activity at the ether methylene group not involving a BVMO reaction was detected as a side reaction with these substrates. The substrates are particularly attractive because they can be prepared in a single step in high yield from commercially available starting materials. They represent to date the only available fluorogenic assay for these enzymes.

1.6 Conclusion

Fluorescence assays for biotransformations are an indispensable tool for enzyme engineering and the daily practice of enzyme studies. Fluorogenic substrates are particularly useful as general probes for enzyme classes that can be used in routine screening and activity checking. However, they cannot replace the authentic substrate in cases where an optimization towards a particular biotransformation is desired. In such cases an indirect fluorogenic assay or an instrumental assay may be required in order to follow the reaction. A variety of fluorescence assays for enzymes still remain to be discovered and the development of new enzyme assays

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Scheme 1.15 Fluorogenic assay for BVMOs in whole cells.

continues to provide fertile ground for innovative developments. One area of recent progress concerns the use of multiple substrates in parallel assays or as mixtures to record activity profiles, also called fingerprints, which enable the characterization of enzyme activities in great detail [59, 60].

Acknowledgment

This work was financially supported by the University of Berne, Swiss National Science Foundation, Swiss Federal Office of Education, COST Program Action D25 on Biocatalysis, and Protéus SA, Nîmes, France.

References 1 Reymond, J.L. (2005) Enzyme Assays: High-throughput Screening, Genetic Selection and Fingerprinting, Wiley-VCH Verlag GmbH, Weiheim. 2 Ravot, G., Wahler, D., Favre-Bulle, O., Cilia, V. and Lefevre, F. (2003) High throughput discovery of alcohol

dehydrogenases for industrial biocatalysis. Advanced Synthesis & Catalysis, 345, 691–694. 3 Klein, G. and Reymond, J.L. (1998) An enantioselective fluorimetric assay for alcohol dehydrogenases using albumincatalyzed beta-elimination of

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Saccharomyces cerevisiae. Tetrahedron Letters, 44, 827–830. Williams, G.J., Domann, S., Nelson, A. and Berry, A. (2003) Modifying the stereochemistry of an enzyme-catalyzed reaction by directed evolution. Proceedings of the National Academy of Sciences of the United States of America, 100, 3143–3148. Woodhall, T., Williams, G., Berry, A. and Nelson, A. (2005) Creation of a tailored aldolase for the parallel synthesis of sialic acid mimetics. Angewandte Chemie – International Edition in English, 44, 2109–2112. Kofoed, J., Darbre, T. and Reymond, J.L. (2006) Dual mechanism of zinc-proline catalyzed aldol reactions in water. Chemical Communications (Cambridge, England), 1482–1484. Kofoed, J., Darbre, T. and Reymond, J.L. (2006) Artificial aldolases from peptide dendrimer combinatorial libraries. Organic & Biomolecular Chemistry, 4, 3268–3281. Kikuchi, K., Hannak, R.B., Guo, M.J., Kirby, A.J. and Hilvert, D. (2006) Toward bi-functional antibody catalysis. Bioorganic & Medicinal Chemistry, 14, 6189–6196. Debler, E.W., Ito, S., Seebeck, F.P., Heine, A., Hilvert, D. and Wilson, I.A. (2005) Structural origins of efficient proton abstraction from carbon by a catalytic antibody. Proceedings of the National Academy of Sciences of the United States of America, 102, 4984–4989. Manetsch, R., Zheng, L., Reymond, M.T., Woggon, W.D. and Reymond, J.L. (2004) A catalytic antibody against a tocopherol cyclase inhibitor. Chemistry, 10, 2487–2506. Zheng, L., Manetsch, R., Woggon, W.D., Baumann, U. and Reymond, J.L. (2005) Mechanistic study of proton transfer and hysteresis in catalytic antibody 16E7 by site-directed mutagenesis and homology modeling. Bioorganic & Medicinal Chemistry, 13, 1021–1029. Janes, L.E. and Kazlauskas, R.J. (1997) Quick E. A fast spectrophotometric method to measure the enantioselectivity of hydrolases. Journal of Organic Chemistry, 62, 4560–4561. Janes, L.E., Lowendahl, A.C. and Kazlauskas, R.J. (1998) Quantitative screening of hydrolase libraries using pH

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indicators: identifying active and enantioselective hydrolases. Chemistry – A European Journal, 4, 2324–2331. Konarzycka-Bessler, M. and Bornscheuer, U.T. (2003) A high-throughput-screening method for determining the synthetic activity of hydrolases. Angewandte Chemie – International Edition in English, 42, 1418–1420. Baumann, M., Sturmer, R. and Bornscheuer, U.T. (2001) A highthroughput-screening method for the identification of active and enantioselective hydrolases. Angewandte Chemie – International Edition, 40, 4201–4204. Wahler, D. and Reymond, J.L. (2002) The adrenaline test for enzymes. Angewandte Chemie – International Edition in English, 41, 1229–1232. Wahler, D., Boujard, O., Lefevre, F. and Reymond, J.L. (2004) Adrenaline profiling of lipases and esterases with 1,2-diol and carbohydrate acetates. Tetrahedron, 60, 703–710. Babiak, P. and Reymond, J.L. (2005) A high-throughput, low-volume enzyme assay on solid support. Analytical Chemistry, 77, 373–377. Grognux, J. and Reymond, J.L. (2006) A red-fluorescent substrate microarray for lipase fingerprinting. Molecular Biosystems, 2, 492–498. Badalassi, F., Wahler, D., Klein, G., Crotti, P. and Reymond, J.L. (2000) A versatile periodate-coupled fluorogenic assay for hydrolytic enzymes. Angewandte Chemie – International Edition in English, 39, 4067–4070. Wahler, D., Badalassi, F., Crotti, P. and Reymond, J.L. (2001) Enzyme fingerprints by fluorogenic and chromogenic substrate arrays. Angewandte Chemie – International Edition in English, 40, 4457–4460. Wahler, D., Badalassi, F., Crotti, P. and Reymond, J.L. (2002) Enzyme fingerprints of activity, and stereo- and enantioselectivity from fluorogenic and chromogenic substrate arrays. Chemistry, 8, 3211–3228. Nyfeler, E., Grognux, J., Wahler, D. and Reymond, J.L. (2003) A sensitive and

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selective high-throughput screening fluorescence assay for lipases and esterases. Helvetica Chimica Acta, 86, 2919–2927. Grognux, J. and Reymond, J.L. (2004) Classifying enzymes from selectivity fingerprints. Chembiochem, 5, 826–831. Grognux, J., Wahler, D., Nyfeler, E. and Reymond, J.L. (2004) Universal chromogenic substrates for lipases and esterases, Tetrahedron, Asymmetry, 15, 2981–2989. Leroy, E., Bensel, N. and Reymond, J.L. (2003) Fluorogenic cyanohydrin esters as chiral probes for esterase and lipase activity. Advanced Synthesis & Catalysis, 345, 859–865. Sicart, R., Collin, M.P. and Reymond, J.L. (2007) Fluorogenic substrates for lipases, esterases, and acylases using a TIMmechanism for signal release. Biotechnol Journal, 2, 221–231. Bensel, N., Reymond, M.T. and Reymond, J.L. (2001) Pivalase catalytic antibodies: towards abzymatic activation of prodrugs. Chemistry, 7, 4604–4612. Leroy, E., Bensel, N. and Reymond, J.L. (2003) A low background high-throughput screening (HTS) fluorescence assay for lipases and esterases using acyloxymethylethers of umbelliferone. Bioorganic & Medicinal Chemistry Letters, 13, 2105–2108. Sicard, R., Chen, L.S., Marsaioli, A.J. and Reymond, J.L. (2005) A fluorescence-based assay for Baeyer–Villiger monooxygenases, hydroxylases and lactonases. Advanced Synthesis & Catalysis, 347, 1041–1050. Yang, Y.Z., Babiak, P. and Reymond, J.L. (2006) New monofunctionalized fluorescein derivatives for the efficient high-throughput screening of lipases and esterases in aqueous media. Helvetica Chimica Acta, 89, 404–415. Yang, Y.Z., Babiak, P. and Reymond, J.L. (2006) Low background FRET-substrates for lipases and esterases suitable for highthroughput screening under basic (pH 11) conditions. Organic & Biomolecular Chemistry, 4, 1746–1754. Badalassi, F., Wahler, D., Klein, G., Crotti, P. and Reymond, J.L. (2000) A versatile periodate-coupled fluorogenic assay for

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hydrolytic enzymes. Angewandte Chemie – International Edition, 39, 4067–4070. Archelas, A. and Furstoss, R. (2001) Synthetic applications of epoxide hydrolases. Current Opinion in Chemical Biology, 5, 112–119. Monterde, M.I., Lombard, M., Archelas, A., Cronin, A., Arand, M. and Furstoss, R. (2004) Enzymatic transformations. Part 58: enantioconvergent biohydrolysis of styrene oxide derivatives catalysed by the Solanum tuberosum epoxide hydrolase, Tetrahedron, Asymmetry, 15, 2801–2805. Zocher, F., Enzelberger, M.M., Bornscheuer, U.T., Hauer, B. and Schmid, R.D. (1999) A colorimetric assay suitable for screening epoxide hydrolase activity. Analytica Chimica Acta, 391, 345–351. Doderer, K., Lutz-Wahl, S., Hauer, B. and Schmid, R.D. (2003) Spectrophotometric assay for epoxide hydrolase activity toward any epoxide. Analytical Biochemistry, 321, 131–134. Mateo, C., Archelas, A. and Furstoss, R. (2003) A spectrophotometric assay for measuring and detecting an epoxide hydrolase activity. Analytical Biochemistry, 314, 135–141. Yang, Y.Z., Wahler, D. and Reymond, J.L. (2003) Beta-amino alcohol properfumes. Helvetica Chimica Acta, 86, 2928–2936. Bedia, C., Casas, J., Garcia, V., Levade, T. and Fabrias, G. (2007) Synthesis of a novel ceramide analogue and its use in a high-throughput fluorogenic assay for ceramidases. Chembiochem, 8, 642–648. Badalassi, F., Nguyen, H.K., Crotti, P. and Reymond, J.L. (2002) A selective

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2 Immobilization as a Tool for Improving Enzymes Ulf Hanefeld

2.1 Introduction

The application of enzymes as catalysts in organic chemistry is closely linked to their immobilization. Indeed, many enzymes are only available in an immobilized form. The immobilized enzymes can be used as received, greatly easing their application. Numerous of these readily available immobilized enzymes are now the working horses of biocatalysis. This has even led to the incorrect use of the abbreviation of an enzyme name for a specific enzyme preparation, that is CALB for the immobilized form of Candida antarctica lipase B on cross-linked polymethacrylate (also known as Novozym 435). Vice versa the commercial name of an enzyme preparation – Amano PS – has taken the place of the enzyme (Burkholderia cepacia lipase on dextrin or diatomaceous earth). Surprisingly, often no attention is paid to the fact that the enzyme is immobilized [1]. Why then is it important whether the enzyme is immobilized and what are the advantages of immobilizing an enzyme? In general, homogeneous catalysts and enzymes in solution have a number of advantages over heterogeneous catalysts (Table 2.1). However, homogeneous catalysts also have drawbacks: in particular it is difficult to separate them from the reaction mixture [1–3]. In addition recycling is problematic and continuous processes are not possible. By immobilizing enzymes several of the advantages of heterogeneous catalysts can be added to the list of advantages of using an enzyme. For enzymes another point can be important. Not all pure enzymes are stable in solution. In particular in organic solvents enzymes can be very unstable [4]. The stability of enzymes can be improved by genetic modifications, by chemical modification, and most easily and without specialist equipment: by immobilization. Furthermore, immobilization opens up the possibility of improving other properties of the enzyme, such as its activity, substrate specificity, and enantioselectivity. Although the underlying principles are not always understood enzyme immobilization is a powerful tool for the improvement of activity and enzyme stability, specificity, and selectivity.


22

2 Immobilization as a Tool for Improving Enzymes Table 2.1 Comparison of homogeneous catalysts and enzymes with heterogeneous catalysts.

Homogeneous catalysts and enzymes

Heterogeneous catalysts

Advantages

Mild reaction conditions High activity and selectivity Efficient heat transfer

Excellent separation Straightforward recycling of the catalyst Continuous processes

Disadvantages

Difficult separations Product contamination No continuous process

Heat transfer problems Low activity and selectivity

When a homogeneous catalyst, that is an enzyme, is immobilized there are in principle three different approaches [3]. The enzyme can be adsorbed onto a carrier, it can be encapsulated in the carrier, or it can be covalently attached to the carrier. In an extreme case of covalent attachment the enzyme can be cross-linked, making a carrier unnecessary. The choice of immobilization method and carrier can greatly influence the properties of the immobilized enzyme, ideally improving them. To date many excellent reviews [5–13] and even comprehensive books [14] on the topic of enzyme immobilization have been written. Many of these reviews and books use their own division of approaches: they range from three to eight and can also include membrane techniques. In order to ease the comparison with chemical catalysts, a division according to reference [3] was chosen. This chapter aims to demonstrate, with examples from the research performed within the COST D25 Action, the importance of the area under discussion.

2.2 Adsorption/Electrostatic Interactions

The adsorption of an enzyme can proceed via different types of interactions. If the enzyme has a large lipophilic surface area, Van der Waals interactions with a hydrophobic carrier lead to the fixation of the enzyme on the carrier. Hydrophilic surface areas or sugar residues attached to the protein can ensure adsorption via hydrogen bonds. In the most extreme case ionic interactions, where the enzyme acts as the counter ion to the carrier, guarantee the binding of the enzyme to the carrier. In all cases the enzyme does not have to be chemically modified and can be employed without pre-treatment. By varying the immobilization conditions the results can be greatly influenced, allowing a straightforward manipulation of the enzyme properties. A significant disadvantage of immobilizing by adsorption is that the enzyme may leach from the carrier under reaction conditions.


23

Lipase Lipase

lid closed

Lipase

lid open, active site accessible

hydrophobic carrier

hydrophilic surface hydrophobic surface

Figure 2.1 Most lipases are interfacially activated. When immobilized on hydrophobic carriers they are assumed to be in their active conformation.

2.2.1 Van der Waals Interactions

The enzymes that are almost by definition most suited for immobilization via Van der Waals interactions are the lipases. Lipases are active at the interface: indeed, many lipases show interfacial activation [15]. It is commonly assumed that those lipases that have a lid covering their active site will only change into the active conformation (lid open and active site accessible) when they are in contact with a lipophilic interface. By adsorbing them onto lipophilic surfaces the lid should ideally be fixed in an open conformation (Figure 2.1). Moreover, as interfacially active enzymes, part of their surface is lipophilic, ensuring the right orientation of the active site during the reaction [6, 9]. Consequently many lipases have been immobilized on hydrophobic carriers. Increases in activity have been reported for several lipases that were immobilized on hydrophobic carriers such as EP-100 polypropylene, Accurel MP1004 polypropylene, octyl-silica, and octyl-agarose [9, 14, 16–20]. This has led to the development of many different applications. In the fermentation broth of lipase producing micro-organisms the lipase is often the only lipophilic component. It was demonstrated that lipases can be extracted from the fermentation broth and purified by adsorption of hydrophobic carriers in one single step [21, 22]. However, this isolation method also reveals one of the disadvantages of immobilization on hydrophobic carriers. After immobilization and removal from the fermentation broth the lipases could be washed off the carrier rather readily. Indeed, when utilizing lipases immobilized on hydrophobic carriers in aqueous media, leaching can be a major problem. If the immobilized enzyme is, however, used in hydrophobic organic solvents, no leaching occurs since the enzyme is insoluble in the solvents. When the lidless CALB is immobilized on a hydrophobic macroporous polymer that is based on methyl and butyl methacrylic esters and cross-linked with divinyl-

24

2 Immobilization as a Tool for Improving Enzymes

Scheme 2.1 Immobilized CALB can be used in organic solvents: it is readily removable and recyclable. Table 2.2 Recycling of Novozym 435 (CALB immobilized on a hydrophobic carrier) in the kinetic resolution of 1c in toluene at 60 °C.

Ratio (% ee)

R-1c S-5c Corresponding aldehyde

Cycle 1

2

3

4

5

98 (99) 86 (90) 16

96 (99) 86 (90) 18

98 (97) 88 (93) 14

98 (98) 90 (93) 12

98 (98) 91 (93) 10

benzene it is called Novozym 435 [23]. This preparation of CALB is extremely successful and popular. Unlike most other lipases CALB and, hence, Novozym 435 is not activated at the interface [15, 24]. When used in toluene for the kinetic resolution of aromatic cyanohydrin acetates (1) the advantage of using an immobilized enzyme could be clearly demonstrated [25]. At the end of the enzymecatalyzed reaction the enzyme could be filtered off and the reaction mixture could be submitted to a second transformation, the chemical protection of the (S)cyanohydrins (2). This yielded both enantiomers of the cyanohydrins, protected with different protection groups and readily separable (Scheme 2.1). Straightforward removal of the enzyme allowed its recycling. After five cycles no loss of


Scheme 2.2 Novozym 435 catalyzes the enantioselective synthesis of aliphatic cyanohydrins via a DKR.

activity or enantioselectivity was observed (Table 2.2), clearly demonstrating the value of immobilization. The same CALB preparation was applied in many dynamic kinetic resolutions; combining two types of catalysts with each other. In the presence of homogeneous transition metal catalysts that catalyze the racemization and heterogeneous acids or bases or immobilized transition metals Novozym 435 was not deactivated [1, 26–28]. This is all the more remarkable since the reactions catalyzed by these catalysts include redox reactions at elevated temperatures (>60 °C). When Novozym 435 was applied for the enantioselective synthesis of cyanohydrin acetates (10) from aliphatic aldehydes (7), good results were achieved (Scheme 2.2) for this dynamic kinetic resolution (DKR) [29]. Here NaCN is used as the base for the dynamic racemic formation and degradation of the cyanohydrins (6 and 8). It has to be noted that CALB has also been immobilized on Accurel. On this carrier its stereoselectivity and activity is even better than when immobilized as Novozym 435. However, it is also stated that diffusion effects need to be taken into account, emphasizing that there can always be more than one reason why an enzyme performs better/worse when modifying the carrier and that the improvements seen might actually have nothing to do with the carrier [30]. Indeed, Novozym 435 and free CALB were more recently shown to have identical stereoselectivity [31]. When looking at the surface of CALB it can be concluded that a large part of it, including the area around the active site, is lipophilic (Figure 2.2) [32]. One might intuitively say that, when immobilized on a hydrophobic carrier, this enzyme should be inactive, since the active site might be inaccessible. It seems that the enzyme instead orients itself in such a way that the active site is still readily approachable. 2.2.2 Hydrogen Bonds

Most enzymes have hydrophilic surfaces, which are often glycosylated. Consequently they can readily form hydrogen bonds with polar surfaces. Therefore they

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26


Figure 2.2 Hydrophobic and hydrophilic surface areas of CALB. Reproduced with permission from [32]. Copyright (2007) Wiley-VCH Verlag GmbH & Co. KGaA.

can easily be immobilized on hydrophilic carriers (cellulose, lignine, Avicel, Celite, porous glass, clay, silica gel) [9, 12, 14]. One such carrier is Celite (diatomaceous earth), the silicate skeletons of diatoms [33]. There are many different types of Celite and care has to be taken to employ the right type. A particularly interesting characteristic of Celite is that it can be used for controlling the water activity in organic solvents, thereby helping to maintain the correct reaction conditions for the enzyme [34]. Celite is a carrier for many commercial lipase preparations. Given the great ease of immobilizing on Celite, it is also used for the immobilization of many different enzymes in the laboratory [35–37]. Often sugars, but also polyethylene glycol (PEG) or albumin, are added in order to stabilize the enzyme and protect it against the negative influence of organic solvents. Hydroxynitrile lyase (Oxynitrilase) from Hevea brasiliensis (HbHNL) immobilized on Celite displayed higher activity and slightly higher enantioselectivity than when immobilized on Avicel (microcrystalline cellulose) or hydrophobic polyamide Accurel EP 700. HbHNL on Celite showed good to excellent stability when stored in dry organic solvents, but was not active when used in dry solvents [37–39]. HNLs from Prunus amygdalus, Manihot esculanta, and Sorghum bicolor were included in the study and it was shown that, for all four HNLs, they are active and selective in organic solvents in the presence of 1–1.5% water, that is with a water layer. When lipases are immobilized on Celite they can readily be used in dry organic solvents. Pseudomonas cepacia (also named B. cepacia) lipase was immobilized in the presence of sucrose on Hyflo super-cel Celite [36, 40, 41] and used in the first enantioselective synthesis of cyanohydrin acetates via a DKR. Similarly, other successful syntheses of cyanohydrin acetates via DKR were catalyzed by lipases immobilized on Celite [42]. This is in contrast to Novozym 435, which had successfully been used for the DKR of aliphatic cyanohydrin acetates (see Section 2.2.1 and Scheme 2.2) [29]. When Novozym 435 was used for the enantioselective synthesis of mandelonitrile acetate (1a) via DKR, the reaction did not proceed [43]. It was


Scheme 2.3 CALB immobilized on Celite R-633 catalyzes the enantioselective synthesis of aromatic cyanohydrins via a DKR.

identified that acetic acid formed during the reaction via hydrolysis of part of the acyldonor (9) neutralized the base that catalyzed the racemic dynamic formation of the cyanohydrin (2). This hydrolysis was caused by residual water that was released from the hydrophobic carrier of Novozym 435. When the carrier was replaced by Celite R-633 CALB proved to be an excellent catalyst for this DKR starting from aromatic aldehydes (Scheme 2.3) [44, 45]. Obviously the Celite maintains a very low water activity in the reaction medium, thereby preventing any hydrolysis and acetic acid formation. As mentioned above, Celite can control water activities in organic solvents, keeping them very low and constant. In a direct comparison under optimized conditions, both enzyme preparations catalyzed this DKR, but still CALB immobilized on Celite was the catalyst of choice (Figure 2.3). The difference in performance can easily be explained by the different orientation of the enzyme on the hydrophobic and hydrophilic carrier (Figure 2.2) [32]. Furthermore, it was recently demonstrated that proteins undergo conformational changes when immobilized and that these changes are dependent on the carrier [46]. Consequently, it would actually be surprising if an enzyme immobilized on different carriers would behave the same on each of these different carriers. 2.2.3 Ionic Interactions

The strongest electrostatic interactions are ionic interactions. The surface areas of enzymes contain amino and acid groups, due to lysine (ε-amino group), aspartic acid, or glutamic acid (second acid group) [13]. Depending on the pH of the solution these groups can bear a charge and can thus interact with an ionic surface. Essentially this type of immobilization can take place on any ion exchanger. The ion exchanger can be positively or negatively charged, depending on the predomi-

27

28


Figure 2.3 Enantioselective synthesis of 1a via DKR catalyzed by CALB on Celite R-633 (䊏) and Novozym 435 (䊐).

nate charge on the enzyme (Figure 2.4) [14]. CALB was immobilized on the anion exchanger polyethylenimine (PEI, containing many amino groups, which are positively charged). It was shown that the pH value and temperature at which the enzyme was immobilized greatly influenced the activity and enantioselectivity of the enzyme [47]. It had earlier already been demonstrated that PEI can also activate lipases that were first bound onto hydrophobic carriers. Obviously PEI interacts with the hydrophilic surface of the lipases and influences their conformation [48]. In addition to anion and cation exchangers as enzyme carriers it has been demonstrated that mixed ion exchange supports can be used for binding enzymes with both acid and amino groups at pH values close to the isoelectric point, such as penicillin G acylase from Escherichia coli (Figure 2.4) [49]. Tailored mesoporous siliceous carriers were prepared in a systematic study of the immobilization of chloroperoxidase from Caldariomyces fumago, an enzyme

2.2 Adsorption/Electrostatic Interactions H3N OOC H3N

NH3

H3N

NH3

OOC COO

H3N H3N

NH3

H3N OOC H3N

NH3

OOC

COO

H2N NH3

OOC H3N

COO

H3N OOC OOC

COO

OOC

COO

H2N NH3

OOC OOC

COO

Figure 2.4 Different ionic interactions between ion exchangers and enzymes.

that nature did not develop for chlorinations [50, 51]. Based on the size of the enzyme (diameter 6.2 nm), the electrostatic surface potential (Figure 2.5a), and its isoelectric point of 4 (Figure 2.5b) the pore size of the carrier was tuned and amino groups were incorporated into the structure of the carrier. The best carrier was a mesoporous material with an average pore diameter of 15 nm with positively charged amino propyl groups to match the negative charge of the chloroperoxidase. At pH 6 virtually all the enzyme was adsorbed on and into the carrier and the immobilized enzyme could be recycled five times before the activity decreased. That ionic immobilization is strongly dependent on the pH value and salt concentrations during immobilization and of the reaction mixture in which the enzyme is used cannot be overemphasized. If these parameters are not taken into

29

30


Figure 2.5 (a) Poisson–Boltzmann electrostatic surface potential of chloroperoxidase at pH 7.0 (blue represents areas of positive charge and red, areas of negative charge) and (b) calculated charge on chloroperoxidase as a function of pH. Reprinted with permission from [51]. Copyright (2007) American Chemical Society.

N

O

enzyme

HN O

O N

2 Ni O

O

O

H N

N

O

Figure 2.6 A carrier fixed Ni2+ acts as anchor for an enzyme with a His6 tag.

account or if in the case of porous materials the wrong pore size is chosen the enzyme will not be immobilized or will leach immediately [52]. Another approach towards immobilization via ionic interactions is the use of tethered metal ions, such as Cu2+, Co2+, or Ni2+, in order to bind the enzyme [53, 54]. This is particularly so when the enzyme contains an easily accessible imidazole residue from histidine [55] or a His6 tag (Figure 2.6) [56–59], that is a short tag with six histidines. This tag can be readily introduced by genetically modifying the enzyme. Little influence of the tag on the catalytic performance has been noticed. When benzaldehyde lyase [60] was immobilized on an Ni2+-containing polyvinylpyrrolidinone-based matrix, it could be reused several times for the formation of benzoin (12) (Scheme 2.4) [58]. Protein-coated micro-crystals (PCMC) are straightforwardly prepared forms of immobilized enzymes. An aqueous solution of the enzyme and a salt such as potassium sulfate is made and a water miscible solvent is added. The salt precipitates and the enzyme forms a layer on top of the micro-crystals. It is thus readily accessible. Given the direct interaction of the protein with the growing crystal of

2.3 Encapsulation

Scheme 2.4 Immobilized BAL with a His6 tag catalyzes the formation of benzoin 12.

Scheme 2.5 Synthesis of sol gels.

the salt the immobilization will most likely proceed via ionic interactions. The PCMCs can only be used in organic solvents and are stable to storage [61].

2.3 Encapsulation

The best way to immobilize an enzyme while affecting its structure as little as possible is to encapsulate it. Out of all the different encapsulation methods the most prominent and widely used is the sol gel technique [5, 7, 62]. Sol gels are highly porous silica materials that are readily prepared and modified (Scheme 2.5). The sol gel obtained is a chemically inert glass that be formed into any desired shape and can be designed to be thermally and mechanically very stable. Most importantly the synthesis proceeds under conditions that are relatively benign for many enzymes. In the first step a tetra-alkoxysilane such as tetramethoxylsilane (TMOS) is hydrolysed by acid catalysis. Hydrolysis is followed by condensation and the sol is formed, which is a mixture of partially hydrolyzed and partially

31

32


condensed monomers. Further condensation leads to the gel. Here all the pores are filled with water and it is therefore also known as aqua gel. This aqua gel can then be dried by evaporation of the water and alcohol. Under these conditions capillary forces will cause the sol gel to shrink significantly, losing some of its structure. The dry gel thus obtained is called xero gel. If the water and alcohol is exchanged against acetone and then against supercritical carbon dioxide the aqua gel can be dried avoiding the capillary forces and, thus, shrinkage of the gel. The aero gel obtained in this manner has a huge pore volume but is rather brittle [5]. The different sol gels obtained in this manner are all hydrophilic. If an alkyltrialkoxysilane such as methyltrimethoxysilane (MTMS) is added to the synthesis mixture, the sol gel prepared will have a hydrophobic surface. Thus, a straightforward change in the synthesis allows a major modification of the gel. The modifications of the surface ensure that the sol gel can either be used as a hydrophilic carrier, which forms hydrogen bonds with the enzyme (see also Section 2.2.2) or as hydrophobic carrier (see also Section 2.2.1). Sol gels with hydrophobic surfaces have a very positive influence on the reactivity of lipases, possibly because they are immobilized in their active conformation (lid open) (Figure 2.1) [63, 64]. Similar to the adsorption approach it is possible to stabilize and activate the enzymes even further by adding surfactants, polyvinylalcohol, or crown ethers. In addition, the sol gel can be mechanically strengthened by including porous glass beads or silica glass fibers at different stages of the sol gel synthesis [64–66]. Given the huge increase in activity that is often observed for lipases in hydrophobic sol gels, HbHNL was also immobilized in a hydrophobic sol gel. HbHNL is structurally closely related to the α/β hydrolases and, thus, the lipases. However, HbHNL was already denatured in the sol gel (Scheme 2.5). When the methanol in the sol gel was removed under vacuum before adding the enzyme, 65% of the HbHNL activity was retained in the aqua gel. Drying to obtain xero gels or aero gels caused complete loss of activity. However, the aqua gels could be used in organic solvents and catalyzed the formation of cyanohydrins with high enantioselectivity (Scheme 2.6) [67]. Based on these results sol gels of PaHNL and MeHNL were prepared and the three HNLs were compared as aqua gel and as free enzyme (Table 2.3) [68]. Strickly

Scheme 2.6 HbHNL encapsulated in an aqua gel catalyzes the enantioselective formation of S-cyanohydrins.

2.4 Covalent Binding/Cross-linking Table 2.3 Conversion ratios and ee’s (parentheses) and

reaction times in the synthesis of mandelonitrile (2a) catalyzed by free HNLs and their corresponding aquagels. (S )-HbHNL

(S )-MeHNL

(R)-PaHNL

Freea

Aqua gelb

Freea

Aqua gelb

Freea

Aqua gelb

4 h: 97(97)

0.5 h: 97(99)

4 h: 97(98)

0.5 h: 96(99)

4 h: 98(97)

2 h: 97(97)

Reaction conditions: benzaldehyde 11a (0.5 mmol/ml diisopropylether), HCN (3 eq), and HNL (6 U/mmol) were shaken at room temperature. a The HNL stock solution was diluted with citrate/phosphate buffer (50 mM, pH = 5.0) to a DIPE:aqueous media ratio of 5 : 1. b DIPE saturated with citrate/phosphate buffer (50 mM, pH = 5.0).

speaking, both reactions are two-phase reactions: the free enzymes are in a buffer layer and so are the immobilized enzymes. However, this time the buffer layer is inside the aqua gel and the system therefore at first glance looks like a reaction performed in organic solvent. The immobilized enzymes perform better than the free enzymes. This might be due to an improvement of the enzymes. However, it is more likely that the large surface area of the buffer layer in the aqua gels enhances phase transfer of benzaldehyde (11a) and mandelonitrile (2a). As already observed earlier, enhancement of the performance of an enzyme upon immobilization is not always due to an improved action of the enzyme [30]. The lipase from B. cepacia (BCL) has successfully been immobilized in xero gels [66] and aero gels [65]. Recently, BCL was immobilized in a xero gel prepared from MTMS and TMOS (Scheme 2.5) and lyophilized. This BCL preparation showed excellent properties for acylations in dry organic solvents [69]. As mentioned above (Section 2.2.2 and Scheme 2.3) residual water in the reaction mixture can cause acyl donors and esters to hydrolyze, thus releasing acid. This in its turn can significantly disturb the desired reaction. When the xero gel-immobilized BCL was compared with Amano PS (a commercial preparation of BCL) it hydrolyzed significantly less of the esters (Table 2.4). Consequently this xero gel is the BCL preparation of choice when reactions are performed in dry media. In addition, the xero gel could be recycled eight times in the synthesis of 14 from the racemic alcohol and vinylacetate. Only modest loss of activity or selectivity was observed (Figure 2.7).

2.4 Covalent Binding/Cross-linking

Covalent binding of an enzyme to a carrier has the advantage that the enzyme is tightly fixed. Thus deactivation due to leaching is no problem and the immobilized enzymes can often be reused. A disadvantage is, however, that the enzyme is

33

34

2 Immobilization as a Tool for Improving Enzymes Table 2.4 Conversion (%) after 24 h of enantiopure esters (0.05 M) by hydrolysis due to residual water in the enzyme preparation in dry organic solvent at room temperature.

Entry

Substrate

Solvent

log P

Conversion (%) by Amano PSa

Conversion (%) by xero gelb

1 2

Toluene TBME

2.8 1.35

4 18

1 3

3 4

DIPE TBME

1.9 1.35

20 12

10 2

5c

TBME

1.35

86

16

a 100 mg/ml of lipase PS powder. b Lyo sol gel based on 100 mg/ml of lipase PS powder. c Reaction temperature 48 °C.

Figure 2.7 Lyophilized xero gel of BCL maintains activity and enantioselectivity in the synthesis of 14 over eight cycles. (䊏) cycle 1, E = 139, (䉱) cycle 8, E = 90.


chemically modified. These modifications are not always easy to direct and in most cases the immobilization is not uniform for all enzyme molecules in one batch. However, enzymes can be bound stably by multipoint attachment and even multimeric enzymes have been efficiently bound on a range of carriers [8, 9, 13]. An extreme case of covalent binding is cross-linking of enzymes. Instead of fixing the enzyme to a carrier, the enzyme acts as a carrier itself. Enzyme aggregates or crystals, enzymes in a spray-dried form, or even enzymes in solution can be cross-linked. The immobilized enzyme is carrier free, that is the material is virtually pure enzyme and the negative effects of carriers can thus be avoided [10, 70]. For the covalent immobilization or cross-linking of enzymes all the functional groups on the enzyme surface can be utilized, including sugar residues [13, 71]. Most commonly the amino groups of the enzyme are employed, either as nucleophile, to attack for instance an epoxide or an aldehyde. In the case of the aldehyde the Schiff base formed can be reduced with NaBH4, thus making the immobilization irreversible. Alternatively carbodiimide can be used for activating an acid group on the enzyme to couple with an amino group of the carrier. The carrier can have hydrophobic or hydrophilic surfaces and the reactive groups can be attached via short or long spacers to the carrier (Figure 2.8). The carriers Eupergit C and Eupergit C 250 L have a reactive epoxy group and are porous acrylic beads [72]. Sepabeads are methacrylic carriers bearing either epoxy or amino groups. The amino groups of the carrier can be linked to the enzyme via glutaraldehyde [32]. Agarose, glyoxyl agarose [73, 74], and the aminated glyoxyl agarose (MANA – the primary amino group is particularly nucleophilic) [75], as well as glutaraldehydemodified agarose or silica [76] have also proven their value as carriers. A bifunctional molecule, such as glutaraldehyde, has to be used for crosslinking, but otherwise the same basic principles apply [10, 11, 70, 71]. An additional possibility that has recently been explored is to first immobilize an enzyme on a carrier and cross-link it afterwards. In this manner particularly stable preparations can be obtained [77]. When Candida rugosa lipase (formerly named Candida cylindracea lipase) was immobilized on an epoxy-activated resin it became resistant against acetaldehyde. Due to this immunization it could be repeatedly employed for the enantioselective acylation of secondary alcohols with vinyl acetate in dry organic solvents (Scheme 2.7) [78]. In a comparative study CALB was immobilized on epoxy-activated Sepabeads and amino Sepabeads with long and short spacers (glutaraldehyde was used as the coupling reagent) (see Figure 2.8). Lyophilized CALB and Novozym 435 were also included in the test. The specific activity (U/g dry immobilized CALB) of Novozym 435 was much lower than for CALB immobilized on the different Sepabeads. In a thermal stability screening, Novozym 435 and CALB immobilized on amino Sepabeads with short spacers displayed equal stability, while all the other CALB preparations were less stable [32]. Cross-linked enzyme aggregates (CLEA) are prepared by first aggregating enzymes via the addition of precipitants such as ammonium sulfate, acetone,

35

36

2 Immobilization as a Tool for Improving Enzymes OH

O

H N

H2N

H NH2

O

O

O

N

H

H

H2N

H

NaBH4 N H

N H

N

N

O NH2 + R

N

O

C

N R' + HOOC

N H

OH

OH +

O

OH

periodate IO4

OH agarose

H

O O

NH2

1.) H2N

O

MANA

carrier

enzyme R

H OH

N

OH O

O N

NH2

2.) NaBH4

glyoxyl agarose

R

H N

HO HO H+

H

H

H OH O OH

HO O H

H H

Figure 2.8 Covalent immobilization can be achieved by linking either amino or acid groups of the enzyme with epoxides, amines, or aldehyde groups on the carrier. In addition, sugar residues can be coupled to the carrier.

O H

OH


Scheme 2.7 Immobilization on an epoxy-activated resin protects C. rugosa lipase against acetaldehyde-induced deactivation.

Scheme 2.8 Preparation of CLEAs.

ethanol, or 1,2-dimethoxyethane. Subsequently a cross-linker, commonly glutaraldehyde, is added (Scheme 2.8). CLEAs of BCL were prepared with bovine serum albumin as the protecting reagent [79] and in the presence of dextrin [69]. The BCL CLEA containing dextrin was significantly more active than the commercial Amano PS preparation of BCL and also more active than the xero gel of BCL (see also Section 2.3) [69]. In addition, the enantioselectivity of BCL is improved when immobilized as a CLEA. However, when the BCL CLEA is submitted to recycling experiments (acylation of 14 with vinyl acetate in dry diisopropylether (DIPE)) the CLEA rapidly loses activity (28% in the second cycle). In comparison the BCL xero gel (Figure 2.7) shows much higher stability and recyclability [69]. For an assessment of different immobilization techniques, CLEAs of HbHNL, PaHNL, and MeHNL were compared with aqua gels of these enzymes [68]. When the CLEAs were used in the presence of a buffer layer; that is under conditions comparable to the aqua gels (see also Section 2.3), all of them were stable [68, 80] and PaHNL can be recycled up to 10 times without loss of activity [81]. The real advantage of the immobilization as CLEAs only becomes obvious when they are employed in pure organic solvents. Here MeHNL displays remarkable stability

37

38

2 Immobilization as a Tool for Improving Enzymes Table 2.5 Conversion ratios and ee’s (parentheses) in the synthesis of mandelonitrile (2a) catalyzed by CLEAs of different HNLs.

(S )-HbHNL CLEA

(S )-MeHNL CLEA

(R)-PaHNL CLEA

72 h: 55(67)

2 h: 96(97)

72 h: 97(99)

Reaction conditions: benzaldehyde 11a (0.5 mmol/ml DIPE containing traces of water from the HCN solution), HCN (3eq), and the respective CLEA (6 U/mmol) were shaken at room temperature.

and reactivity (Table 2.5, compare with Table 2.3). Immobilization of MeHNL as a CLEA enables its use in pure organic solvents, thus expanding the scope of the enzyme significantly [68, 82].

2.5 Conclusion

The immobilization of enzymes has been extensively studied during the last decades [14]. Many approaches have been investigated and it is now possible to improve the stability of enzymes in such a way that they can be used under harsh, non-natural conditions. Organic solvents, supercritical fluids, and ionic liquids (Chapters 5 and 8) [83] do not constitute an insurmountable problem any longer. At the same time, enzyme immobilization has helped to improve the activity of enzymes, in particular that of lipases. In addition to the activity the selectivity and enantioselectivity of the enzymes could be modified and improved. However, although all these steps have been undertaken the approach is still an iterative experimental improvement, not a theory-based choice of ‘the right method’. Given the huge variety in structure between the enzymes, indeed already between the different isoenzymes of one enzyme, the vast choice of carriers and the different binding methods there is – to date – no general solution to all problems.

Acknowledgments

The author would like to thank all the students, PhD students, and postdoctorals, past and present, for their work that led quite unintended into the field of immobilization. The contacts stimulated, established, and strengthened during the COST D25 Action, significantly helped in undertaking large parts of the research described here. In particular L. Gardossi (Trieste), P. J. Halling (Strathclyde), L. T. Kanerva (Turku), E. Magner (Limerick), F. Molinari (Milan), and A. Pierre (Lyon) are gratefully acknowledged for introducing the author to many different aspects of enzyme chemistry.

References

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47 Torres, R., Ortiz, C., Pessela, B.C.C., Palomo, J.M., Mateo, C., Guisan, J.M. and Fernandez-Lafuente, R. (2006) Enzyme and Microbial Technology, 39, 167–171. 48 Guisan, J.M., Sabuquillo, P., FernandezLafuente, R., Fernandez-Lorente, G., Mateo, C., Halling, P.J., Kennedy, D., Miyata, E. and Re, D. (2001) Journal of Molecular Catalysis B: Enzymatic, 11, 817–824. 49 Fuentes, M., Batalla, P., Grazu, V., Pessela, B.C.C., Mateo, C., Montes, T., Hermoso, J. A., Guisan, J.M. and Fernandez-Lafuente, R. (2007) Biomacromolecules, 8, 703–707. 50 Murphy, C.D. (2006) Natural Product Reports, 23, 147–152. 51 Hudson, S., Cooney, J., Hodnett, B.K. and Magner, E. (2007) Chemistry of Materials, 19, 2049–2055. 52 Essa, H., Magner, E., Cooney, J. and Hodnett, B.K. (2007) Journal of Molecular Catalysis B: Enzymatic, 49, 61–68. 53 Wang, F., Guo, C., Liu, H.-Z. and Liu, C.Z. (2007) Journal of Molecular Catalysis B: Enzymatic, 48, 1–7. 54 Osman, B., Kara, A., Uzun, L., Besirli, N. and Denizli, A. (2005) Journal of Molecular Catalysis B: Enzymatic, 37, 88–94. 55 Hochuli, E., Döbeli, H. and Schacher, A. (1987) Journal of Chromatography, 411, 177–184. 56 Augé, C., Malleron, A., Tahrat, H., Marc, A., Goergen, J.-L., Cerutti, M., Steelant, W.F.A., Delannoy, P. and Lubineau, A. (2000) Chemical Communications, 2017–2018. 57 Nahalka, J., Liu, Z., Chen, X. and Wang, P.G. (2003) Chemistry – A European Journal, 9, 373–377. 58 Dräger, G., Kiss, C., Kunz, U. and Kirschning, A. (2007) Organic and Biomolecular Chemistry, 5, 3657–3664. 59 Cassimjee, K.E., Trummer, M., Branneby, C. and Berglund, P. (2008) Biotechnology and Bioengineering, 99, 712–716. 60 Sukumaran, J. and Hanefeld, U. (2005) Chemical Society Reviews, 34, 530–542. 61 Kreiner, M., Moore, B.D. and Parker, M.C. (2001) Chemical Communications, 1096–1097. 62 Avnir, D., Braun, S., Lev, O. and Ottolenghi, M. (1994) Chemistry of Materials, 6, 1605–1614.

References 63 Reetz, M.T., Zonta, A. and Simpelkamp, J. (1995) Angewandte Chemie – International Edition in English, 34, 301–303. 64 Reetz, M.T., Wenkel, R. and Avnir, D. (2000) Synthesis, 6, 781–783. 65 Orcaire, O., Buisson, P. and Pierre, A.C. (2006) Journal of Molecular Catalysis B: Enzymatic, 42, 106–113. 66 Reetz, M.T., Tielmann, P., Wiesenhöfer, W., Könen, W. and Zonta, A. (2003) Advanced Synthesis Catalysis, 345, 717–728. 67 Veum, L., Hanefeld, U. and Pierre, A. (2004) Tetrahedron, 60, 10419–10425. 68 Cabirol, F.L., Hanefeld, U. and Sheldon, R.A. (2006) Advanced Synthesis Catalysis, 348, 1645–1654. 69 Hara, P., Hanefeld, U. and Kanerva, L.T. (2008) Journal of Molecular Catalysis B: Enzymatic, 50, 80–86. 70 Cao, L., van Rantwijk, F. and Sheldon, R.A. (2000) Organic Letters, 2, 1361–1364. 71 Schoevaart, R., Siebum, A., van Rantwijk, F., Sheldon, R. and Kieboom, T. (2005) Starch, 57, 161–165. 72 Boller, T., Meier, C. and Menzler, S. (2002) Organic Process Research & Development, 6, 509–519. 73 Guisan, J.M. (1988) Enzyme and Microbial Technology, 10, 375–382. 74 Mateo, C., Palomo, J.M., Fuentes, M., Betancor, L., Grazu, V., Lopez-Gallego, F., Pessela, B.C.C., Hidalgo, A., Fernandez-Lorente, G., Fernandez-

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3 Continuous-flow Microchannel Reactors with Surface-immobilized Biocatalysts Malene S. Thomsen and Bernd Nidetzky

3.1 Introduction

Microstructured reaction technologies offer fundamental advantages to synthetic chemistry [1]. Besides the benefits of miniaturization for high-throughput reaction optimization [1b], the atypical fluid behavior in microchannels with inner dimensions of between several tens to hundreds of micrometers provides a uniquely useful and defining feature of microstructured reactors, in comparison with conventional macroscale reactors [1c, 2]. Fluid flow is mostly laminar, directed, and symmetric, thus facilitating continuous reaction engineering and reactor design. Improved control of key process parameters such as flow rate or temperature can result in enhanced yield and improved chemical selectivity of chemical conversions. Although mass transport occurs by diffusion, the high surface area-tovolume ratio of microchannels (≈10 000 m2/m3 or greater) often allows conversions to be performed in the chemical regime where the rate of transport is high compared with the reaction rate. The use of microstructured reactors opens up possibilities for new production concepts, most notably continuous processing and flexible scale-up on demand via parallelization (numbering-up) [2, 3]. While microstructured reactors have gained widespread acceptance as tools of synthetic chemistry [1], biocatalytic transformations in microfluidic systems are not advancing as quickly ([4] and references therein). Although enzymes are often integrated into microfluidic ‘lab-on-a-chip’ systems for analytical applications, the development of scalable microstructured reactors for enzyme-catalyzed conversions has attracted comparably little attention so far. This is surprising considering that microwell systems, featuring parallelized shaken vessels for batch and fed-batch operation in the microliter scale, are now widely in use for miniaturized bioprocess studies [5]. Not only would a microstructured flow reactor facilitate screening of candidate enzymes from natural biodiversity or mutant libraries, but it should also provide an excellent system for biocatalytic process analysis and design. Experimentation in a suitable microstructured reactor under flow conditions could allow one to collect process information early during the


44

3 Continuous-flow Microchannel Reactors with Surface-immobilized Biocatalysts

development which in turn would have a direct impact on reducing the time to market [5a]. To be useful as a technology, continuous flow processing in microstructured biocatalytic reactors must integrate recycling of the enzyme used most conveniently and efficiently via immobilization ([4] and references therein). One possibility is to attach the enzyme onto microparticles and assemble the functionalized beads in the microchannel as a miniature fixed bed. The complex liquid flow pattern and the high differential pressure required for obtaining practical flows are possible disadvantages. A second option is to immobilize the enzyme directly on the microchannel walls (for a review, see reference [4]; for examples of coatedwall microreactors using chemical catalysts, see references [1b, 2, 6]). Although the specific surface area available for the reaction may be relatively small in this case, a microfluidic reactor featuring enzyme-coated channels would seem to be a useful tool for biocatalytic microprocess technology. After briefly reviewing the state of the art, this chapter will summarize the results of microreactor development for enzymatic hydrolysis of lactose by the hyperthermostable β-glycoside hydrolase CelB from Pyrococcus furiosus [7].

3.2 Biocatalytic Synthesis Using Microreaction Technology with Free and Immobilized Enzymes

Recent reviews have provided systematic coverage of the enzymatic microreactors used in chemical analysis [4]. Considering that the focus of this chapter is biocatalytic synthesis, it does not consider the analytical applications and the reader is referred to the cited literature ([4] and references given therein). The use of microreactors for high-throughput kinetic characterization of enzymes is another very interesting application of the technology [8], which, for reasons of limited space, is not discussed herein. Kanno et al. [9] reported on on-chip synthesis of oligosaccharides using glycosidase-catalyzed transgalactosylation. The authors did not use immobilized enzyme. However, a remarkable result of their study was that the product yield could be enhanced significantly in comparison with the conventional batch conversion. Other hydrolytic enzymes, namely lipase [10] and protease [11] were also tested in microchannel reactors. Different approaches of entrapment were used for enzyme immobilization. The results showed that the yields of product could be improved and that the requirements for reactant were reduced through the use of microreactors. Jones et al. [12] designed an enzyme reactor in which urease was immobilized on the microreactor wall by incorporating the enzyme directly into the polydimethylsiloxane (PDMS) material used for microstructure fabrication. Belder et al. [13] performed enantioselective catalysis using a soluble epoxide hydrolase and analysis on a microfluidic chip. NAD(H)-dependent enzymatic redox reactions were also carried out in microfluidic systems [14]. Methods for electrochemical regeneration of NADH were described [14b]. Oxidative enzymatic

3.3 Novel Microfluidic Immobilized Enzyme Reactors

reactions examined in microchannel systems include degradation of p-chlorophenol catalyzed by laccase [15] and hydroxylation of an alkaloid catalyzed by a bacterial P450 enzyme [16]. More recently, Ku et al. [17] reported polyketide synthesis and functionalization using two microchannel reactors in series. The first reactor contained type III polyketide synthase immobilized onto agarose microbeads via interactions of the protein ‘his tag’ and Ni-nitrilotriacetic acid groups displayed on the surface of the particles. The second reactor contained soybean peroxidase attached on the surface of the microchannel. The study showed the rapid evaluation of different reaction conditions for the two-step enzymatic conversion and the effect of the varied conditions on the final product structure. Microstructured multi-enzyme reactors may become more widely used in the future for combinatorial synthesis. Honda et al. [18] introduced the technology of cross-linked enzyme aggregates to microstructured reactors. They deposited an aggregate formed between an aminoacylase and poly-L-lysine in the presence of glutardialdehyde on the inner wall of a silica capillary. The enzyme reactor thus obtained showed good operational stability and the activity of the immobilized enzyme was more resistant to thermal stress and organic solvents than that of the free enzyme. In a more recent paper Honda et al. [19] integrated an immobilized aminoacylase microreactor with a microdevice for two-phase liquid extraction to perform continuous enzymatic resolution of racemic amino acid derivatives with in situ product removal. Luckarift et al. [20] reported on multistep chemo-enzymatic synthesis of 2-aminophenoxazin-3-one using a continuous flow microdevice in which several microfluidic chips were connected in series. Silica-bound immobilized enzymes were used in the reaction.

3.3 Novel Microfluidic Immobilized Enzyme Reactors 3.3.1 Microreactor Design

Two different microstructured reactors for use with immobilized enzymes have been developed. The first device integrated a microstructured multichannel plate fabricated by micro-injection molding from a two-component liquid silicon rubber material (Silopren LSR 4070) with an appropriately interfaced and temperature-controlled housing, as shown in Figure 3.1a. The plate was designed to contain nine linear microchannels, an inlet zone that ensured equal division of liquid into the channels, and an outlet zone (Figure 3.1b). Each microchannel was characterized by a length of 64 mm, a width of 350 µm, and a height of 250 µm. Bas-relief microstructures were incorporated every 2.5 mm along each microchannel, alternating between the left and right walls (Figure 3.1c). They were included as flow obstacles to improve mass transfer to and from the microchannel surface through a passive mixing effect [1b, c, 2, 3].

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Figure 3.1 Microreactor featuring a multichannel microfluidic element fabricated from PDMS [21]. Panel a: the fully assembled microreactor. Panel b: microstructured multichannel plate. Panel c: electron micrograph of a segment of a microfluidic channel that shows a passive mixing element.

The liquid volume accommodated by the PDMS plate was 167 µl. The microfluidic element was placed in a housing designed as a sandwich-like construction consisting of two plates made of poly(methyl methacrylate), as can be seen in Figure 3.1a. One plate was transparent and contained inlet and outlet for fluid flow. The other element consisted partly of poly(methyl methacrylate) and an embedded metal plate. It was used for temperature control from an external water bath. The microreactor was designed for use under conditions of pressure-driven continuous fluid flow, which was delivered from a high-pressure liquid chromatography (HPLC) pump (Knauer Smartline 1000). Optionally, the outlet of the device was connected to a flow-through absorbance detector [21]. The second device was a multiplate-stacked microfluidic reactor, originally developed by Hessel, Löwe and co-workers for gas-phase transformations [1b] (Figure 3.2) and now adapted for biocatalytic processing with immobilized enzymes. It is termed GPMR (gas phase microreactor) throughout. The microfluidic element of the microreactor was a stainless steel plate possessing 34 linear channels (Figure 3.2b). Its inlet was interfaced with the multichannel domain in order

3.3 Novel Microfluidic Immobilized Enzyme Reactors

Figure 3.2 GPMR used for biocatalytic transformations with immobilized enzymes [22]. (a) the fully assembled microreactor, (b) microstructured multichannel plate, and (c) electron micrograph of the wash-coat layer of γ-aluminum oxide covering the microchannel walls.

to achieve proper fluid equipartition over the microchannels, each of which had a length of 20 mm and was characterized by an aspect ratio of 1.5 (width 300 µm and height 200 µm). The microreactor could be assembled with up to 10 microreaction plates, each accommodating a liquid volume of 24.5 µl and stacked alternating with microheat-exchanger plates. The walls of each channel were wash coated with a 10–20 µm layer of γ-aluminum oxide (Figure 3.2c), which allowed random tethering of the enzyme via chemical coupling [23]. Note that, according to literature reports [24], the γ-aluminum oxide coating of the microchannels does not influence the residence time distribution of the continuously operated microreactor. The inlet of the microreactor was connected to the HPLC pump mentioned above. The outlet was connected with an Upchurch Micro-Splitter Valve from Postnova Analytics (Landsberg, Germany) to enable manual back pressure regulation, typically maintained at 0.5–1.0 MPa. 3.3.2 Enzyme Immobilization

A useful compilation of methods employed for the immobilization of enzymes on microchannel surfaces is given by Miyazaki and Maeda [4a]. We used glutardialdehyde-mediated cross-linking of amino groups on the microstructured plates with amino groups of the enzyme. The surface area available for attachment of protein on the PDMS plate was estimated to be 793 mm2. Due to the presence of the γ-aluminum oxide coating it is difficult to know the exact value for the relevant surface area on the GPMR plate. From the number and the geometry of the microchannels on the microstructured plate, a minimum surface area of 340 mm2 was calculated. The surface of PDMS is hydrophobic which results in poor wettability with aqueous solvents and promotes non-specific protein adsorption. It is also relatively inert to chemical modification [25]. The liquid silicon rubber chosen for fabrication of the reaction plate contained pyrogenic silicic acid as a filler. Aside from its effect on elastomer properties the silicic acid can be expected to provide additional silanol

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groups for surface chemistry. The microfluidic plate was silanized using in-flow treatment with an aqueous solution of (3-aminopropyl)triethoxysilane at pH 3.5– 4.0. The silanized plate was then derivatized with glutardialdehyde solution. In the final step, the pre-activated plate was incubated with enzyme solution, typically containing about 0.1 mg protein/ml. A full description of the protocol used has been published elsewhere [21]. Enzyme immobilization on the GPMR plate took place by an analogous procedure with the exception that a pH of 7.0 was used during the silanization step to prevent destruction of the wash-coat layer of γaluminum oxide [22]. Immobilization experiments involved recombinant β-glycosidase CelB from P. furiosus produced in Escherichia coli. A partially purified preparation obtained from the soluble protein fraction of a heat-treated E. coli cell was employed [7]. The specific activity of CelB towards lactose was about 800 units/mg protein at 80 °C. We were able to immobilize about 60 µg and up to 100 µg of CelB on the PDMS and GPMR plates, respectively.

3.4 Enzymatic Hydrolysis of Lactose 3.4.1 Catalytic Effectiveness of Immobilized CelB

The enzyme activity bound on the reaction plate was measured under continuous flow conditions at 80 °C. Substrate solution typically containing 600 mM of lactose in 20 mM sodium citrate buffer at pH 5.5 was delivered at different flow rates and the concentration of glucose in samples collected at the outlet of the microreactor operated in the steady state was measured off-line. Figure 3.3 shows typical results. The enzyme activity corresponds to the slope of the linear relationship between the lactose converted and mean residence time τav, which is the ratio of the total reactor volume (24.5 µl per plate, a volume determined by the supplier (Institut für Mikrotechnik Mainz GmbH) and the flow rate (Figure 3.3)). CelB tethered onto GPMR plates retained about 50% of the specific activity (= activity bound/ protein bound) of the free enzyme. By contrast, the specific activity of CelB immobilized on the PDMS plate was only about 3% that of the free enzyme. By way of comparison, CelB attached onto silanized macroporous glass beads using conditions otherwise exactly identical to the ones employed for immobilization of the enzyme on microstructured plates, showed about 35% of the original specific activity. 3.4.2 Continuous Conversion of Lactose

Figure 3.4 summarizes the results of microreactor experiments in which the effect of a varied flow rate on the enzymatic conversion of lactose was analyzed. The

3.4 Enzymatic Hydrolysis of Lactose

Figure 3.3 Determination of the activity of immobilized CelB under conditions of continuous flow [22]. The slope of the linear dependence of product concentration on residence time (τav) was used to calculate the activity ∆CV/∆ταϖ where V is the reactor

volume. Reactions were performed in the GPMR at 80 °C using 600 mM of lactose (pH 5.5) as the substrate. A single microreaction plate was used (V = 24.5 µl). It contained a total amount of 13 U of enzyme activity.

Figure 3.4 Comparison of the performance of the immobilized enzyme microreactors during continuous conversion of 100 mM of lactose (80 °C and pH 5.5) [21, 22]. GPMR (full circles); PDMS microreactor (full squares).

Reactions were performed using a volumetric enzyme activity of 82 U/ml (GPMR, two stacked plates, and total volume of 49 µl) and 6 U/ml (PDMS microreactor and total volume of 167 µl).

average residence time required to hydrolyze ≥60% of the initial concentration of lactose (100 mM) was 4 min in the GPMR and about eight times lower than in the PDMS reactor. However, the comparison of the two microreactors must take into account that the concentration of active enzyme per milliliter of reactor working volume used in the GPMR (≈500 U/ml) was 56-fold higher than that in the PDMS reactor (≈12 U/ml). Therefore, this implies that, after correction for the effect of enzyme loading, the catalytic performance of the PDMS reactor was about 7 (=56/8) times more efficient than that of the GPMR.

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Figure 3.5 Operational stability of the immobilized enzyme microreactors during continuous conversion of 100 mM of lactose (80 °C and pH 5.5) [21, 22]. GPMR (full circles); PDMS microreactor (full squares). Reactions were performed using a τav value

of 3.2 min (GPMR) and 33 min (PDMS microreactor). The volumetric enzyme activity was 184 U/ml in the GPMR (two stacked plates and total volume of 49 µl) and 12 U/ml in the PDMS microreactor (total volume of 167 µl).

In order to examine the operational stability of CelB in each of the two microreactors, we carried out time course experiments, the results of which are shown in Figure 3.5. The average residence time for lactose hydrolysis in the GPMR reactor (3.2 min) and the PDMS reactor (33 min) was constant and chosen to enable conversion of 60–70% of the initial concentration of substrate present, assuming a fully stable enzyme. Under these conditions, a decrease in the steadystate level of released glucose during the time span of each experiment, typically 100 h, would indicate gradual loss of activity. Both enzyme reactors showed useful stability.

3.5 Biocatalytic Process Intensification Using Microreaction Technology

The term process intensification encompasses a multilevel design approach aiming at the improvement of key features of chemical processing, such as size reduction of equipment, a decrease in energy consumption, and in safety of operation while achieving a given production objective [26]. Microreaction technology is a promising tool with which to achieve process intensification through matching of the fluid dynamics of a process with the physicochemical requirements of a reaction [2, 27]. According to an analysis performed for heterogeneously catalyzed gas-phase transformations [2, 27a], coated-wall microchannel reactors hold great potential for eventually outperforming conventional fixed-bed technology with respect to the above-mentioned criteria of intensified chemical processing. In particular, the characteristic time scale for a chemical transformation is expected to vary with microchannel diameter (dM), from about dM1 to dM2 for reaction

3.6 Conclusions and Outlook

rate-limited to diffusion-limited conversions, respectively. Therefore, chemical processing with immobilized catalysts appears to be quite suitable for miniaturization to the microscale and the general idea is strongly supported by an increasing number of case studies of catalytic gas- or liquid-phase reactions [1b, 2]. A rigorous chemical engineering analysis that compares the performance of the microstructured device with that of a relevant macro fixed bed reactor is presently lacking for immobilized biocatalytic systems. The potential benefit of equipment size reduction when using microreaction technology can be exemplified through space–time yield (STY) comparison. A STY of about 62 g/(l h) was reported for a continuously operated fixed bed reactor that utilized processing at 70 °C with CelB immobilized onto Eupergit C particles (90 units/ml of reactor) [7a]. The linear flow rate used with the fixed bed reactor was 0.64 cm/min, being in the range of linear flow rates (0.125–400 cm/min) employed with the microreactor. The STY obtained with the immobilized CelB microreactor (530 units/ml of reactor) operated at 80 °C was 500 g/(l h). Therefore, when taking into consideration the 25% decrease in enzyme activity caused by lowering the temperature from 80 to 70 °C (data not shown), the comparison of the two reactors reveals that, after normalization in the used enzyme loading to 1 kU CelB/l, the performance of the microfluidic device and the packed bed reactor was almost identical with 0.71 g glucose/lh and 0.69 g glucose/lh, respectively. This shows that the miniaturization to the microfluidic format concomitant with a change from catalytic particle to catalytic wall geometry does not enhance the intrinsic productivity of lactose conversion into glucose and galactose catalyzed by immobilized CelB. The benefits of introducing the enzyme into the microfluidic system was in this case the higher volumetric productivity obtained. Critical analysis of the performance of the different CelB reactors provides a note of caution in showing that reactor miniaturization to a microfluidic format alone is clearly not sufficient to improve the productivity of enzymatic lactose conversion. The STY of the continuously operated multichannel microreactor fabricated from PDMS was only 20 glucose/lh in spite of the fact that the available microstructured surface area was close to that of a two-plate stacked steel microreactor. Retention of just 3% of the specific activity of soluble CelB in the enzyme preparation immobilized onto PDMS explains the relatively much lower value for STY. The importance of selection of a suitable surface for enzyme immobilization is stressed and a washcoat layer of γ-aluminum oxide seems to be a promising candidate.

3.6 Conclusions and Outlook

Two types of bioreactors are commonly used in industry for performing enzymecatalyzed transformations: the stirred tank reactor and the packed or fluidized bed reactor [28]. Substitution of these well-established reactors by a system based on microfluidic technology will happen only if there are clear and compelling advantages in so doing. It is therefore necessary to consider, in a rigorous and

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case-specific manner, the potential assets of microstructured reactors for biocatalytic synthesis other than serving as a tool during initial process development in the laboratory. No clear and generally valid answers can be given at the present time. However, the following general benefits could be anticipated based on the results summarized herein and obtained in other groups [1–4]: intensification of processing, especially for multiphase reactions, easier integration of unit operations like biocatalytic reaction and product removal, by liquid–liquid extraction for example [19], ability to carry out multistep organic syntheses in the absence of intermediate purification, easier implementation of kinetically controlled reactions, change of product properties, faster transfer of research results into production and therefore earlier start of production at lower costs, and, finally, easier scale-up of production capacity.

Acknowledgements

We gratefully acknowledge the contributions from A. P. Zurk (dbs – daily business solutions GmbH) in the fabrication of the microstructured PDMS reactor and from Dr D. Kirschneck (Microinnova GmbH) in the assembly and set-up of the GPMR. Dr P. Pölt and C. Elis from the Institute for Electron Microscopy at Graz University of Technology kindly performed measurements by scanning electron microscopy. Work from the senior author’s laboratory was supported by a grant from the Austrian Federal Ministry of Transport, Innovation, and Technology (Program ‘Factory of Tomorrow’, Project 807947). Dr D. Kirschneck, Dr M. Koncar (VTU Engineering), and Mag. H. Reichl (Hämosan) were industry partners in the project. They are thanked for their encouragement and for financial support.

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4 Activity and Stability of Proteases in Hydrophilic Solvents Lars Haastrup Pedersen, Sinthuwat Ritthitham and Morten Kristensen

4.1 Introduction

Proteases and lipases have proven to be very efficient biocatalysts in both hydrophobic and hydrophilic solvents. The choice of enzyme and solvent system opens up a wide range of possibilities in process design for manipulating the selectivity and solubility of reactants and products. When it comes to specific transformations and regioselective substitution of complex carbohydrates, lipids, and peptides biocatalysis can be used for producing acceptable yields at relatively mild reaction conditions. Therefore biocatalysis offers a strong alternative to conventional chemistry, avoiding the need for protection and deprotection steps required in chiral and regioselective organic synthesis. In contrast to lipases, proteases do not catalyze direct esterification with fatty acids as acyl donors, but proteases have proven to be efficient catalysts of transesterification reactions with carbohydrates ranging in size from mono- to polysaccharides as acyl acceptors. Proteases catalyze the esterification of carbohydrates using activated esters as acyl donors. Members of the subtilisin family (S8) have been shown to catalyze specific acylation of sucrose at the C-1′ position (see Figure 4.1), whereas thermolysin (EC 3.4.24.27) catalyzes the acylation of sucrose primarily at the C-2 position as well as regioselective acylation of the glucose moieties of cyclodextrins. This regioselectivity differs from most lipases including the ones from Humicola lanuginosa and Pseudomonas sp., which catalyze the acylation of sucrose primarily at the C-6 position and to a minor extent both at the C-6, C-6′ positions and the C-6, C-1′ positions. Taking advantage of the differences in regioselectivity allows for esterification at specific positions of the carbohydrate. Thus, by controlling both the acyl chain length and the positional distribution of fatty acids on the sugar molecule, systematical changes of the physico-chemical properties of these amphiphilic biosurfactants are obtained. In order to achieve an optimal process the solvent must solubilize the reactants, while at the same time maintaining enzyme activity. For synthesis of biosurfactants hydrophilic solvents are good solvents of both hydrophilic carbohydrates and hydrophobic acyl donors, but are known as denaturants of most proteins. Reduced Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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4 Activity and Stability of Proteases in Hydrophilic Solvents

Figure 4.1 Important disaccharides.

activity and complete inactivation of lipases in hydrophilic, aprotic solvents at concentrations of 20–30% (v/v) has been observed [1–3]. In contrast, alkaline proteases have proven active and relatively stable under these conditions. Therefore the stability of these enzymes in organic solvents has been investigated and attempts to relate stability characteristics to protein structure and conformation have been made. Protease- and lipase-catalyzed sugar ester synthesis has been reviewed by Plou et al. [4] and Kennedy et al. [5], whereas specific reviews on proteases in organic solvents and industrial utilization of alkaline proteases were compiled by Bordusa [6] and Gupta et al. [7], respectively. The aim of the present chapter is to give a brief overview of protease-catalyzed synthesis of sugar esters in hydrophilic solvents and to present some of the most recent investigations on the effect of these solvents on activity and stability of proteases. Consequently, the perspectives in solvent engineering are outlined with focus on hydrophilic solvents and ionic liquids.

4.2 Activity and Selectivity of Proteases in Synthesis of Carbohydrate Fatty Acid Esters

The catalytic properties and regioselectivity of proteases of the subtilisin family including subtilisin Carlsberg have been thoroughly investigated in transestrification reactions with trihaloethyl esters and vinyl esters of short- to medium-chained fatty acids and a range of mono- and disaccharides as acyl acceptors. Anhydrous pyridine and N,N-dimethylformamide (DMF) have been shown to be good solvents for these reactions both in terms of solubility of the substrates and sustained activity of the alkaline protease. Some of the most abundant hexoses were all substituted at the primary hydroxyl group at the C-6 position (see Table 4.1). However, by adding dimethyl sulfoxide (DMSO) to the reaction medium the regioselectivity

4.2 Activity and Selectivity of Proteases in Synthesis of Carbohydrate Fatty Acid Esters

57

Table 4.1 Selectivity of alkaline proteases in the synthesis of monosaccharide-based biosurfactants.

Alkaline protease

Solvent and conditions

Acyl donor

Acyl acceptor

Position of substitution

Yield %

Reference

Subtilisin Bacillus subtilis

DMF 45 °C, 18 h

Trichloroethyl butanoate

D-glucose

6

64

[8]

Bacillus sp. (Proleather)

Pyridine 45 °C, 2 days

Trichloroethyl propanoate Vinyl acetate Vinyl benzoate

D-glucose

6

5.2

[9]

D-glucose

6 6

31 33

Divinyl adipate

D-glucose

6 6 6 6

66 63 54 74

6 6 6

30 46 21

2

49

Streptomyces sp.

DMF 35 °C, 7 days

D-glucose

D-galactose D-mannose

α-methyl-Dgalactose DMF : DMSO 4 : 1 35 °C, 7 days

Divinyl adipate

D-glucose D-mannose

α-methyl-Dgalactose D-galactose

in a Streptomyces sp. protease-catalyzed reaction was shifted from the primary hydroxyl to the secondary hydroxyl group at the C-2 position of galactose [10]. In both pyridine and DMF subtilisin Carlsberg and other subtilisins showed selectivity towards acylation of primary hydroxyl groups of important disaccharides (see Figure 4.1). Hence, monoesters exclusively substituted at the C-6′ position were obtained with cellubiose and maltose, whereas with lactose the major product was the 6′-O-monoester, but other monoesters substituted at either the C-3′ or the C-4′ position were detected in smaller amounts. The overall substitution pattern clearly reveals regioselectivity towards the primary hydroxyl group situated at the non-reducing end of these three reducing sugars. Using proteinase N as biocatalyst in either DMF or DMSO and water (7% v/v) as co-solvent, sucrose monoesters substituted at each of the three primary hydroxyl groups were obtained, respectively, with the 1′-O-ester as the major product [11, 12]. Sucrose monoesters were also synthesized in anhydrous DMF and pyridine, respectively with different acyl donors and other subtilisins as biocatalysts – in all cases 1′-O-monoester was the major product (see Table 4.2). Using vinyl acrylate as the acyl donor Park and Chang [14] also observed the formation of two different acrylic acid esters of sucrose (1′,6′-di-O-acryloyl- and 6,1′-di-O-acryloyl-sucrose) both substituted at the C-1′ position in combination with one of the two remaining primary hydroxyl groups. With the alkaline protease AL 89 in a mixture of DMF, DMSO, and water (7% v/v) the regioselectivity towards the C-2 position of sucrose was observed.

[10]

58


Table 4.2 Selectivity of alkaline proteases in the synthesis of disaccharide-based biosurfactants.

Alkaline protease

Subtilisin B. subtilis

Solvent and conditions

DMF at 45 °C for 2–7 days

Acyl donor

Trichloroethyl butanoate

Acyl acceptor

Sugar ester yield (%)

Maltose Cellubiose Lactose

50

Substitution pattern position

Reference

distribution mol/mol (%)

6′ 6′ 6′ 4′ 3′

75 10 10

1′

90

[13]

1′ 6, 1′ 1′, 6′

70 18 12

[14]

90 10 10 75

[11]

[8]

Subtilisin Carlsberg Bacillus licheniformis

DMF at 45 °C

Trifluoroethyl butanoate

Sucrose

Subtilisin Carlsberg B. licheniformis (Optimase M-440)

Pyridine at 30 °C for 5 days

Vinyl acrylate

Sucrose

Proteinase N B. subtilis

DMF : H2O 13.3 : 1 at 45 °C for 1 day DMF : H2O 24 : 1 at 45 °C for 3 days

Trifluoroethylmethacrylate

Sucrose

15

Vinyl esters of fatty acids (C8, C10 and C12)

Sucrose

17 (C8)

1′ 6′ 6 1′

40 (C10)

6′

10–13

21 (C12)

6

7–11

[12]

Protease AL 89 Bacillus pseudofirmus

DMSO : DMF : H2O 6.2 : 6.2 : 1 at 45 °C for 1 day

Vinyl-laurate (C12)

Sucrose

57

2

[15]

B. subtilis

Pyridine at 50 °C for 5 days

Divinyl esters of fatty acids (C4, C6 and C10)

Maltose

53 (C4) 42 (C6) 34 (C10) 62 (C4) 46 (C6) 31 (C10) 55 (C6)

6′

[16]

Lactose

Sucrose

6′

1′

4.3 Enzyme Stability and Conformation

Figure 4.2 Sucrose containing biodegradable polymers. 1′-Ovinyl hexanedioyl-sucrose co-polymerized with either acrylic acid or acrylamide [21].

Thus, using vinyl laurate as the acyl donor the enzyme catalyzed the synthesis of 2-O-lauroyl sucrose [15]. The neutral protease thermolysin was shown to have the same regioselectivity with sucrose as the acyl acceptor in anhydrous DMSO [17] and, depending on the chain length of the acyl donor, the glucose moiety in cyclodextrins was substituted at the C-2 or both the C-2 and C-3 or C-6 positions [18]. The carbohydrate fatty acid esters described above have a wide range of applications due to their physical and chemical properties. Particularly laurate monoesters of mono- and disaccharides have anti-microbial properties [19] and vinyl esters of disaccharides can be used for polymerization – either homopolymerized or co-polymerized with acrylic acid or arylamide to produce hydrogels [20, 21]. In this way Wang et al. [21] synthesized biodegradable homopolymers of 1′-O-vinyl hexanedioyl-sucrose and 6′-Ovinyl hexanedioyl-lactose respectively, in which the disaccharide is linked to the polyvinyl main chain via the ester linkage to the hexanedioyl spacer. Furthermore 1′-O-vinyl hexanedioyl-sucrose was co-polymerized with either acrylic acid or acrylamide (see Figure 4.2). Fatty acid esters of cyclodextrins have been used for producing self-assembling nanoparticles for drug delivery [22].

4.3 Enzyme Stability and Conformation

The conformation and dynamics of secondary structural elements of subtilisin Carlsberg in 100% DMSO was studied by Fourier-transform infrared (FTIR) spectroscopy and suggested, on one hand, random coil formation at room temperature

59

60


due to complete unfolding [23]. On the other hand, formation of aggregates or intermolecular β-sheets at 30 °C resulting in very low catalytic activity has been reported [24]. At DMSO concentrations of 54–56% only a partial destruction of the tertiary structure of subtilisin was observed by FTIR and circular dichroism. This structural change induced an enhanced conformational flexibility of the enzyme resulting in an increase in both hydrolytic activity and enantioselectivity towards (S)-ethyl-2-(4-substituted phenoxy) propanoates. Changing the DMSO concentration from 0 to 55% (v/v) the enantiomeric ratio (E-value) increased by a factor 2.4–4.5 depending on the chain length of the alkyl substituent. With a racemic mixture of ethyl-2-(4-hexyl phenoxy) propanoate in 65% (v/v) DMSO an E-value of 53 was obtained, whereas in buffer the E-value was 5.4 [25]. A similar effect caused by DMF on a papaya-type plant cystein protease was observed by Quiroga et al. [26], namely a simultaneous increase in hydrolytic activity and partial unfolding of the enzyme in a mixture of DMF and Tris–HCl buffer pH 8 (1 : 1 v/v). The structural analysis was based on FTIR revealing a decrease in the total content of β-sheet and β-turn secondary structural elements during 4 h of incubation. At a DMSO concentration of 30% (v/v) Ruiz and De Castro [27] reported that the halo-alkalophilic protease from the archaeon Natrialba magadii showed a residual activity of approximately 80% after 7 days of incubation at 30 °C, 1.5 M NaCl, pH 8 and pH 10, respectively. At suboptimal salt concentrations the presence of DMSO had a stabilizing effect on the enzyme. Ogino and Ishikawa [28] compared the hydrolytic activity half-life of four different proteases as effected by a range of solvents. DMSO (25% v/v) had a stabilizing effect on α-chymotrypsin and the two alkaline proteases, subtilisin Carlsberg and Pseudomonas aeruginosa protease PST-01. In fact the latter proved to be extremely stable in DMSO and a range of alcohols showing activity half-lives longer than 50 days. DMF destabilized both α-chymotrypsin and thermolysin, but had a stabilizing effect on the alkaline proteases, subtilisin Carlsberg in particular. The same effect was observed with methanol, but instead of subtilisin Carlsberg it was PST01 showing the highest stability. In terms of stability of the alkaline proteases, water was found to be the worst solvent. Of the more hydrophobic solvents, toluene had a stabilizing effect on all four proteases (see Table 4.3). Klibanov [30] and co-workers [31, 32] showed that the tertiary structure of subtilisin Carlsberg was nearly the same in water, acetonitrile, and dioxane and that the differences observed were of the same magnitude as those observed between structures in different aqueous systems. In particular the active site structures were essentially the same in all three solvents, but also the solvents did not substantially affect the exposed surface area of the enzyme, which was prepared from slightly cross-linked crystals. Recent investigations using circular dichroism have suggested that protein stability was closely related to specific elements of secondary structure with α-helical structures remaining unchanged – whereas β-pleated sheet structures were affected by the addition of methanol. Hence, the relatively high content of α-helical structural elements and low content of β-sheet of PST-01 (see Table 4.4) was seen as

4.3 Enzyme Stability and Conformation Table 4.3 Activity half-life of proteases in the presence of

organic solvent [28, 29]. Values in parentheses are the results of repetitions carried out in 2007. Enzymes were incubated at 30 °C and pH 8 in the presence of 25% organic solvent. Residual hydrolytic activity was measured by a casein assay. Solvent

Water DMSO DMF Methanol Ethanol Acetone Tert-butanol Heptanol Toluene

Log P

−1.35 −1.01 −0.74 −0.30 −0.24 0.35 2.62 2.73

Half life (days) α-Chymotrypsin

Thermolysin

Subtilisin

PST-01

13.2 (0.05) 33.6 2.2 6.0 (0.006) 27.0 0.6 0.5 3.8 >100

10.8 (17.3) 2.6 0.9 4.6 (4.1) 3.0 0.7 0.8 13.1 22.5

0.3 (0.4) 6.4 39.8 26.2 (20.3) >50 24.8 41.6 8.6 5.7

9.7 (10.2) >50 25.3 >50 (41.0) >100 23.1 >50 >50 12.0

Table 4.4 Secondary structural elements composition of

proteases [29]. The compositions were estimated from circular dichroism spectra of enzyme solutions at 30 °C and pH 8. Protease

α-helix (%)

β-sheet (%)

α-Chymotrypsin Thermolysin Subtilisin PST-01

7 38 30 37

30 17 16 13

important factors for the increased half-life observed for this protease in 25% methanol (see Table 4.3), but also the presence of two disulfide bonds contributed to the stability characteristics of PST-01 [29]. The stabilizing effect of introducing new disulfide bonds was demonstrated in a thermolysin-like double mutant, where enhanced thermostability was clearly related to the stabilization of a 14 amino acid long flexible loop region (56–69) by the introduction of a disulfide bond [33]. A closely related mutant, boilysin, proved resistant to boiling with a half-life of 170 min at 100 °C [34]. Both enzymes were mutated far from the active site and therefore the flexibility of the amino acid residues involved in catalysis was not affected. Stabilization against thermal inactivation simultaneously led to increased stability in organic solvents. Although the difference in thermal stability between the two mutants was clear (10 K), no significant difference in the stability towards organic solvents was observed [35].

61

62


Figure 4.3 Dissolution of alkaline protease AL 89 suspended in hydrophilic solvents (M. Kristensen and L.H. Pedersen, unpublished). DMF t½: 29 min; DMSO t½: 6 min. A crude preparation of alkaline protease AL 89 (10 g/l) was suspended and incubated in anhydrous DMF or DMSO at 40 °C. OD600 was measured in triplicates of 200 µl sample volumes in a Tecan microplate reader with continuous shaking.

The effect of DMSO on the initial reaction rate of a protease PST-01-catalyzed synthesis of Z-aspartame in a mixture of DMSO and Tris–HCl buffer at pH 8 was investigated by Tsuchiyama et al. [36]. The initial reaction rate decreased with increasing DMSO concentration (20–70% (v/v)). At 50% (v/v) DMSO an 83% yield of aspartame precursor was obtained and at DMSO concentrations higher than 60% a precipitate assumed to be the protease was observed. We investigated the dissolution behavior of a lyophilized crude preparation of the alkaline protease AL 89 in anhydrous DMSO and DMF, respectively. At first a turbid suspension was obtained in both cases, then the suspension was completely solubilized following an exponential decrease in turbidity (OD600), with the fastest decrease observed in DMSO (see Figure 4.3). During this process the residual hydrolytic activity of the enzyme suspension was analyzed and shown to decrease, correlating with the decrease in turbidity. These observations indicated that the highest activity was maintained as long as the enzyme preparation was in suspension and that solvation caused inactivation. DMSO dissolved the enzyme suspension faster than DMF corresponding to the decrease in residual activity (see Figure 4.4). The optimal balance between substrate solubility and sustained enzyme activity can therefore be found by adjusting the mixture ratio between these two solvents.

4.4 Solvent Engineering

Figure 4.4 Residual activity as effected by dissolution of alkaline protease AL 89 in hydrophilic solvents (M. Kristensen and L.H. Pedersen, unpublished). Enzyme incubated in anhydrous DMF (squares) or DMSO (triangles) in the absence (filled symbols) or presence (open symbols) of 0.2 M sucrose.

Relative turbidity (OD600) was used as a measure of the degree of solubilization of suspended alkaline protease AL 89 (see the description and progression curves in Figure 4.1). Residual hydrolytic activity of 20 µl samples diluted in buffer 1 : 50 was measured by an azo-casein assay at pH 10 and 50 °C.

4.4 Solvent Engineering

Enzyme activity is generally significantly lower in anhydrous organic solvents than in water, not just because of reduced conformational mobility, but also because of factors directly related to the physical–chemical properties of the organic solvent, namely unfavorable energetics of substrate desolvation and transition state destabilization [30]. These effects can often be compensated for by an appropriate choice of organic solvent. Therefore solvent engineering and investigation of new types of reaction media will still be important research areas in relation to biocatalysis. In this context ionic liquids have recently received increasing attention. Ionic liquids are low melting point (98R 95R >98R >98R 97R

>100 >100 52 >100 >100 80

Pentadecane

Scheme 6.1 Enantioselective esterification of (R,S)flurbiprofen with 1-octanol.

6.5 Stereoselective Esterifications of Racemic Carboxylic Acids

Figure 6.3 Molar conversion and enantioselectivity of the esterification of (R,S)-flurbiprofen with 1-octanol catalyzed by dry mycelia (10 g/l) of A. oryzae MIM in toluene at 50 °C. Empty squares, lysate of mycelia; black squares, intact mycelia.

(Figure 6.3). No relevant variations were observed in terms of the reaction kinetics, whereas a slight decrease in the maximum enantioselectivity was detected in the lysate. At 50 °C the maximum conversion (about 55%) was achieved with 84% e.e. of the unreacted (S)-flurbiprofen. It is noteworthy that molar conversion was complete, although during the direct esterification some water was produced. By removing biomass by centrifugation and selectively extracting the acid from the organic solvent with a saturated solution of NaHCO3, recovery of the unreacted flurbiprofen and its separation from the ester were easily accomplished.

87

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6 Direct Esterification with Dry Mycelia of Molds

The results obtained with dry mycelia represent a significant improvement as compared to the data obtained using two commercial preparations of lipase B from Candida antarctica (CAlB) as biocatalysts [22, 23].

6.6 Partition Phenomena and Equilibrium of Esterification Reactions

The high molar conversion obtainable in esterifications catalyzed by dry mycelia encouraged us to investigate the partition of water in these heterogeneous systems further. The objective was to verify whether mycelia were able to affect the thermodynamic equilibrium of the reactions by modifying the partitioning of the water formed during the esterification. The synthesis of two esters by lyophilized mycelia of R. oryzae CBS 112.07 was studied and the results were compared with those obtained with a commercial immobilized CALB (Novozym 435). Table 6.6 reports on the molar conversions of the esterifications of geraniol with hexanoic acid and of cinnamyl alcohol with butyric acid obtained using these two biocatalysts under different conditions. In order to exclude possible adsorption of reagents and/or products on the biocatalysts, a procedure of extensive rinsing of the biocatalysts was applied and the liquid phases were analyzed by high-performance liquid chromatography. No appreciable presence of products nor reagents was observed. The effective achievement of the equilibrium was confirmed in all cases by the addition of further fresh biocatalyst after the reactions stopped. In all cases the conversions achieved by using R. oryzae CBS 112.07 were slightly higher, although both systems led to >90% conversions. In order to understand whether the observed differences were ascribable to any effect of the mycelia on the reaction equilibrium, a series of measurements was aimed at monitoring the variations in the water vapour pressure in the gas phase, expressed as relative humidity (RH). A hygrometer specifically conceived for providing fast measurements in systems involving organic solvents was used for this.

Table 6.6 Molar conversions at equilibrium in the esterifications catalyzed by R. oryzae CBS 112.07 and Novozym 435®.

Solvent

Temperature (°C)

Esterification of geraniol with hexanoic acid Toluene 50 Hexane 30 Esterification of cinnamyl alcohol with butyric Toluene 50

Molar conversion at equilibrium (%) R. oryzae CBS 112.07

Novozym 435®

93 97 acid 95

91 90 91

6.6 Partition Phenomena and Equilibrium of Esterification Reactions

It must be underlined that, in those cases where the system had reached an apparent equilibrium (constant reading for at least 1 h), such values were considered as an acceptable approximation of the aw value of the system. Firstly, the water content of the two dry biocatalysts was evaluated and it was found that 11% of the water was still present in the mycelia after lyophilization (weight variation after 6 h in an oven at 110 °C). The two biocatalysts were then equilibrated in toluene for 24 h and the amounts were chosen so as to obtain the same water content in both systems. The results of Table 6.7 indicate that the mycelia led to a lower water activity, hence suggesting that water might partition inside the mycelia. In order to verify whether water produced during the esterification was released in the bulk solvent or sequestrated inside the cell, samples were prepared where the mycelia wre incubated in toluene in the presence of amounts of water equal to the amount that would be formed in corresponding 50 and 100% of molar conversion of the esterifications studied. The variation in RH in the gas phase was measured for 7 days as reported in Figure 6.4a and b. Moreover, to exclude any possible interference of the procedure in opening the vials upon the insertion of the hygrometer for the reading (empty squares in Figure 6.4a and b), similar experiments were performed where the measurements were done only at the start of the incubation and then after 1 week. A long-lasting process of water partitioning and equilibration leads to values of RH of about 0.3, whereas the theoretical water activity at the end of the reaction for 100% molar conversion would be 0.12 (as calculated by the UNIFAC method). Therefore, these observations suggest that the mycelia contribute marginally to the water take-up. Rather than sequestrating water inside the cell wall, it seems that the mycelia provide a micro-environment where the water produced during the esterification is promptly removed. The profiles of Figures 6.4 and 6.5 indicate that the water formed during the esterification is promptly released into the bulk solvent (RH of about 0.75). The esterification of butyric acid with three different alcohols was studied: cinnamyl alcohol, 1-phenyl-1-butanol, and Z-L-Ser-OBz (Figure 6.5).

Table 6.7 Initial water content of the dry biocatalysts and

water activity measured in a closed system obtained after incubating 60 mg of mycelia and 200 mg of Novozym 435® for 24 h in 6 ml of toluene. Biocatalyst

Water content of the dry biocatalyst (%, w/w)a

aw in tolueneb

R. oryzae CBS 112.07 Novozym 435®

11 3

0.15 0.22

a Difference in weight between the wet and the dried sample after drying samples at 110 °C for 6 h. b Measured using hygrometer DARAI (Trieste, Italy).

89

90

6 Direct Esterification with Dry Mycelia of Molds

Figure 6.4 Variation in the RH in systems constituted of toluene (2 ml), mycelia (10 g/l), and water corresponding to (a) 50% and (b) 100% molar conversion of direct esterifications performed starting from

equimolar substrate concentrations of 25 mM and 50 mM, respectively. Empty squares indicate the values measured in identical experiments where no intermediate reading was performed.

Figure 6.5 Variation in the RH measured during the esterification reaction of butyric acid with different alcohols: cinnamyl alcohol (䉬), 1-phenyl-1-butanol (•), and Z-L-SerOBzl (䉱). Experimental conditions: 2 ml of toluene, 20 mg of lyophilized mycelia, and 50 mM equimolar substrates in a 6 ml sealed vial.

References

After 7 days the conversion was nearly quantitative only in the case of cinnamyl alcohol, whereas in the other two cases the conversion was below 5%. By monitoring the variation in the RH in the gas phase of the three systems it was observed that a significant increase in the RH values was associated only with the reaction corresponding to cinnamyl alcohol: the RH value then decreased with equilibration of the system, which appeared to be very slow.

6.7 Conclusions

Dry mycelia of molds can be effectively used for (stereo) selective esterification of different alcohols and carboxylic acids in organic solvent with notable advantages, such as the following.

• • • •

High stability in organic solvents and high resistance to the inactivation due to free carboxylic acids, including acetic acid. High molar conversions also enabled by favorable partition of water. Good enantioselectivity also permitted by favorable partition of water. Easy set-up of continuous membrane-bioreactor.

References 1 Faber, K. (2004) Biotransformations in Organic Chemistry, 5th edn, Springer Verlag, Berlin. 2 Bell, G., Blain, J.A., Paterson, J.D.E., Shaw, C.E.L. and Todd, R.J. (1978) FEMS Microbiology Letters, 3, 223–228. 3 Gancet, C. and Guignard, C. (1986) Biocatalysis in Organic Media, Elsevier, Amsterdam. 4 Molinari, F., Marianelli, G. and Aragozzini, F. (1995) Applied Microbiology and Biotechnology, 43, 967–973. 5 Bornscheuer, U.T. and Kazlauskas, R.J. (1999) Hydrolases in Organic Synthesis, Wiley-VCH Verlag GmbH, Weinheim. 6 Schipper, M.A.A. and Stalpers, J.A. (1984) Studies in Micology, CBS, Baar. 7 Beer, H.D., Bornscheuer, U.T., McCarthy, J.E.G. and Schmid, R.D. (1998) Biochimica et Biophysica Acta, 1399, 173–180. 8 Langrand, G., Rondot, N., Triantaphylides, C. and Baratti, J. (1990) Biotechnology Letters, 12, 581–586.

9 De Castro, H.F., De Oliveira, P.C. and Pereira, E.B. (1997) Biotechnology Letters, 19, 229–232. 10 Claon, A.C. and Akoh, C.C. (1993) Biotechnology Letters, 15, 1211–1216. 11 Molinari, F., Gandolfi, R., Zilli, M. and Converti, A. (2000) Enzyme and Microbial Technology, 27 (8), 626–630. 12 Gandolfi, R., Converti, A., Pirozzi, D. and Molinari, F. (2001) Journal of Biotechnology, 92, 21–26. 13 Converti, A., Del Borghi, A., Lodi, A., Palazzi, E., Gandolfi, R. and Molinari, F. (2002) Biotechnology and Bioengineering, 77, 232–237. 14 Converti, A., Del Borghi, M., Gandolfi, R., Molinari, F., Palazzi, E. and Zilli, M. (2002) World Journal of Microbiology & Biotechnology, 18, 409–416. 15 Converti, A., Del Borghi, A., Gandolfi, R., Molinari, F., Palazzi, E., Perego, P. and Zilli, M. (2002) Enzyme and Microbial Technology, 30, 216–223.

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6 Direct Esterification with Dry Mycelia of Molds 16 Garcia, H.S., Malcata, F.X., Hill, C.G.Jr, and Amundson, C.H. (1992) Enzyme and Microbial Technology, 14, 535–545. 17 Garcia-Alles, L.F. and Gotor, V. (1998) Biotechnology and Bioengineering, 59, 163–170. 18 Molinari, F., Mantegazza, L., Villa, R. and Aragozzini, F. (1998) Journal of Fermentation and Bioengineering, 86, 62–64. 19 Romano, D., Ferrario, V., Molinari, F., Gardossi, L., Sanchez Montero, J.M., Torre, P. and Converti, A. (2006) Journal of Molecular Catalysis B: Enzymatic, 41, 71–74.

20 Alcantara, A.R., Sanchez Montero, J. M. and Sinisterra, J.V. (2000) Stereoselective Biocatalysis, Marcel Dekker, New York-Basel. 21 Grösch, S., Schilling, K., Janssen, A., Maier, T.J., Niederberger, E. and Geisslinger, G. (2005) Biochemical Pharmacology, 69, 831–839. 22 Spizzo, P., Basso, A., Ebert, C., Gardossi, L., Ferrario, V., Romano, D. and Molinari, F. (2007) Tetrahedron, 63, 11005–11010. 23 Morrone, G., Nicolosi, G., Patti, A. and Piattelli, M. (1995) Tetrahedon: Asymmetry, 6, 1178–1773.

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7 Factors Affecting Enantioselectivity: Allosteric Effects Elisabeth Egholm Jacobsen and Thorleif Anthonsen

7.1 How to Provide Enantiopure Compounds

Many naturally occurring compounds are chiral and often enantiopure. They may be carbohydrates, amino acids, terpenoids, hydroxy carboxylic acids, or alkaloids. They can serve as building blocks for the synthesis of chiral compounds. In these compounds nature has already provided the chirality since natural compounds are synthesized by enzyme catalysis. Selectivity is an important issue in most organic syntheses. It is common to distinguish between selectivity on three levels: chemoselectivity is to discern between reactions at chemically different groups of a molecule such as between a hydroxy group and an amino group, regioselectivity is the discrimination of reactions at sites with chemically equivalent groups but in chemically different environments such as the various hydroxy groups of a carbohydrate, And, thirdly; discrimination of reactions at chemical groups that only differ in stereochemical environment. Enzyme catalysis is able to discern between all of these three: however, stereo discrimination is the most demanding one. The goal of enzyme-catalyzed stereo discriminating synthesis is to produce enantiomerically pure compounds. This target can be reached by two different routes, either by asymmetric synthesis or by resolution of a racemic mixture. These two methods have their particular characteristics. By asymmetric synthesis, apart from reactions based on chiral natural products, the substrate is achiral and the stereochemical principle is inherent in the catalyst, the enzyme. The enzyme is able to catalyze a reaction with an achiral substrate to yield excess of one single enantiomer of a product. The theoretical yield is 100% of product with 100% enantiomeric excess (e.e.). The e.e. of the product is in theory constant throughout the reaction, that is it is independent of the degree of conversion. On the contrary, kinetic resolution gives at the most 50% of each enantiomer and the e.e. of the two enantiomers depends on the degree of conversion (see Figure 7.1). However, by using a deracemization technique such as dynamic kinetic resolution or in situ inversion, the yield and e.e. may be 100%.


94

7 Factors Affecting Enantioselectivity: Allosteric Effects

Figure 7.1 The graph shows how the e.e. values of the substrate (solid lines) and the product (dotted lines) change during the conversion of a racemic mixture. The E-value is set to 50 and calculations have been performed for three different equilibrium constants, where K = 10 000 corresponds to an irreversible reaction. In this situation the

substrate curve touches the 100% line at 55% conversion, which means that enantiopure substrate can be obtained provided that a 5% yield can be sacrificed. The curves for K = 3 and 0.5 show that reversibility in particular, influences the e.e. of the substrate in an unfavorable manner.

7.1.1 Kinetic Resolution of Racemic Mixtures Catalyzed by Enzymes

A common way of producing enantiopure compounds is by resolution. Kinetic resolution of racemic mixtures is often performed by catalysis using hydrolytic enzymes. One characteristic feature of kinetic resolution is that the e.e. of both the product and the remaining substrate depend on the degree of conversion of the reaction [1]. This is in contrast to resolution via diastereomers or by asymmetric synthesis. Since the e.e. of the substrate fraction will sooner or later during a reaction reach 100%, this goal can always be reached, provided some loss of yield is acceptable. This is a characteristic and a clear advantage of kinetic resolution, when the yield is not the most important factor of the reaction, but the e.e. The crucial parameter for a kinetic resolution reaction is the enantiomeric ratio E, commonly called the enantioselectivity factor [2]. Enantioselectivity is, according to the IUPAC, ‘the preferential formation in a chemical reaction of one stereoisomer over another’ [3]. Enantioselectivity is most commonly used for asymmetric synthesis when a prochiral substrate is converted into a mixture of enantiomers in which one enantiomer predominates. However, according to the definition, enantioselectivity can also be used for kinetic resolution in which two enantiomers are converted to product at a different rate. In this case the reaction may also

7.1 How to Provide Enantiopure Compounds

be called enantiomer selective or enantiospecific since the enzyme interacts differently with two different substrates, the two enantiomers. According to the IUPAC, ‘a reaction is termed stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products’. This definition should also cover kinetic resolution since the two enantiomers give different enantiomeric products. The E-value of kinetic resolution is considered to be constant throughout a resolution reaction [4a]. The E-value is basically the ratio of the specificity constants for reactions with the two enantiomers [5]. If, let us say, that the E-value is 19, then, for instance, the R-enantiomer reacts 19 times faster than the S-enantiomer. This means that, at the very start of the reaction, the product will contain a 19 : 1 ratio of the enantiomers or 95% of R and 5% of S, i.e. the e.e. is 90% [6]. 7.1.2 Absolute Configurations in Resolution

The E-value does not in general provide information on which enantiomer is the faster reacting. However, information on the stereo preference of several lipases has been collected and it seems that most lipases follow the so-called Kazlauskas rule [7]. For secondary alcohols the structure of the faster reacting enantiomer of an enantiomeric pair is the one shown in Figure 7.2. It is important to realize that whether this has the R or S configuration depends on the CIP rules. (see Scheme 7.1 for relevant examples. When R = Br or Cl the R/S notation changes, compared to ‘small’ or ‘large’ relative size. Provided that the E-value is high (e.g. E ≥ 20) it can be used for predicting or even be taken as a proof of absolute configuration [8].

Figure 7.2 The structure of the faster reacting enantiomer in lipase-catalyzed esterification in kinetic resolution of racemic secondary alcohols or hydrolysis of the corresponding esters. ‘Small’ and ‘large’ refer to the relative size of the groups and not to the R/S notation.

Scheme 7.1

95

96


7.2 Factors Affecting the Enantiomeric Ratio E

The enantioselectivity of a biocatalytic resolution or asymmetrization is primarily dependent on the enzyme and the structure of the substrate. Both of these can be changed in order to optimize the selectivity. The enzyme can be optimized by molecular genetic methods, while the substrate can be modified by organic chemical synthesis. These ways of optimizing selectivity will not be discussed in this chapter. 7.2.1 Is the E-value Really Constant?

The E-value should be calculated by measuring the e.e. of the remaining substrate and the product at different degrees of conversion [9]. However, it is common to measure the e.e. at one single degree of conversion, but it can also be done by using computer programs utilizing several measurements [10, 11]. The E-value is considered to be constant throughout a resolution and only dependent on the substrate and the enzyme. However, it has been discovered that the E-value may change during the reaction [12]. This feature can only be discovered if the E-value is measured at several degrees of conversion. If computer programs are used for calculating E based on several measurements, an average E will result. The E-value depends as mentioned primarily on the enzyme and the substrate, but also on the acyl donor used in acyl transfer reactions in organic solvents [13, 14]. Acyl donors like activated haloalkyl esters can give high E-values, but may be reversible to some extent. This is in particular harmful for the e.e. of the substrate fraction (see Figure 7.1). Enol esters, on the other hand, such as vinyl or isopropenyl esters, give irreversible reaction conditions, but may reduce the E-value due to acylation of amino groups of the enzyme [13]. The liberated acetaldehyde, when vinyl esters are used as acyl donors, may react with amino groups of the enzyme and, thus, reduce the selectivity [4b], but this is not always a problem [14]. The E-value also depends on the medium in which the reaction is carried out. In a hydrolysis reaction, this is usually water or a buffer, whereas for transesterification reactions this can be almost any organic solvent or mixture of solvents, including some water. During the reaction the medium will change, not because the actual solvent is changing, but because the enantiomers of the substrate are disappearing at a different rate and the enantiomers of product are formed in unequal quantities [15]. They also constitute the medium of the reaction and, consequently, this change can influence the selectivity of the reaction. 7.2.2 Influence of the Reaction Medium on the E-value

It is well recognized that the medium of the resolution reaction can influence the selectivity. The medium is primarily the solvent, which in hydrolysis mainly


consists of water. In a transesterification reaction, however, the medium is organic solvents with varying amounts of water in it. The most relevant way to express the amount of water in the medium is by the water activity aw. The aw value can be set by adding a mixture of inorganic salts with different hydration numbers to the reaction [16]. It has been shown that enantiomer selectivity in resolutions can be increased when the water activity is increased [17]. The effect depends on which organic solvent is used, but when the water activity becomes too high, the acyl transfer reaction will stop. 7.2.3 Influence of Enzyme Immobilization on the E-value

Immobilization of enzyme catalysts is performed primarily in order to improve the technical properties of enzymes. It can be performed in various ways. Two distincly different methods are used: coupling to a carrier or cross-linking or by entrapment in a matrix or by membranes [18]. There are only a few reports on the dependence of selectivity. Reports claim that selectivity can be enhanced [19] or that it is not influenced. It seems reasonable to anticipate that each specific enzyme will behave differently and that more investigations are needed [20]. 7.2.4 Enzyme Inhibition

Inhibition of lipases, both by the substrate or the product, has been observed. In alcoholysis of methyl propanoate with n-propanol catalyzed by Candida antarctica lipase B (CALB), the alcohol was found to inhibit the enzyme resulting in a deadend complex [21]. Phosphate- and phosphonate-containing inhibitors are known to inhibit proteases. Studies of the inhibition of CALB have shown inhibition by diethyl p-nitrophenyl phosphate. The inactivation of the enzyme was caused by covalent binding of diethyl p-nitrophenyl phosphate in the active site [22]. 7.2.5 Enantioselective Inhibition and Activation: Allosteric Effects

The addition of small molecules has been shown to change the enantioselectivity of certain enzyme-catalyzed reactions. It is believed that such molecules bind to a site in the protein different from the active site, which leads to a conformational change in the active site. Such enzymes are called allosteric enzymes, i.e. enzymes that comprise of multiple subunits and multiple active sites. Binding of a cosubstrate or small molecule may cause an increase or decrease in the activity or selectivity of the enzyme. Since enantiomer selectivity, the E-value, is the ratio of the reaction rate with the two enantiomers: either an allosteric increase in the reaction rate with one enantiomer or an allosteric decrease in the reaction rate with the other disfavored one will lead to an increase in E.

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Enantioselective inhibition of Candida rugosa by dextromethorphan and levomethorphan resulting in an enhanced enantiomer selectivity has been reported in kinetic resolution 1989 [23]. Kinetic inhibition experiments have revealed that the molecular action of the base was a non-competitive inhibition (i.e. binding of the base to an allosteric site in the lipase) that caused the inhibition of the transformation of one enantiomer leading to the relative increase in the transformation rate of the other enantiomer [23]. L-Methioninol was found to increase the hydrolysis rate of the (R)-enantiomer in hydrolysis of 3-acetoxy nitriles catalyzed by lipase PS (Pseudomonas sp.) while it inhibited the hydrolysis of the (S)-enantiomer. It was suggested that the substrate and L-methioninol were combined to the enzyme at different sites and that conformational changes due to the binding of L-methioninol provided a change in the affinity towards the substrates [24]. Asymmetrization of 3-(3,4-dichlorophenyl)glutarate with immobilized CALB (Chirazyme L2, Roche) showed a loss of 30% activity within 18 h of the reaction, which was assumed to be due to product inhibition [25]. The decrease in enantioselectivity in CALB-catalyzed resolutions of secondary butanoates observed by Lundhaug et al. [26] was also suggested to be caused by inhibition of the enzyme by the enantiopure product alcohol. Akeboshi and co-workers [27] also reported that the enantioselectivity gradually declined, accompanying the progress of the hydrolysis of benzyl-protected primary alcohols and that the alcohol product inhibited the hydrolysis rate of the faster reacting enantiomer. This phenomenon was first reported in the 1930s when it was found that strychnine enhanced human liver esterase-catalyzed hydrolysis of methyl L-mandelate but not the hydrolysis of the D-isomer. These results also indicated an apparent allosteric binding of the enantiopure additive [28, 29]. Organic bases have also been reported to increase the enantioselectivity of lipase-catalyzed reactions in water-saturated organic solvents [30, 31]. It is known that both the activity and E-value of CALB (Novozym 435) increased dramatically upon addition of triethylamine (Et3N) to the water saturated organic reaction medium in a reaction between 2-phenyl-4-benzyloxazol-5(4H)-one and butan-1-ol. It was believed that the base was able to make an ion pair with the acidic byproduct, thus avoiding the inhibitory effect of the acid by dissolving it and thereby removing it from the micro-environment of the enzyme [32]. The addition of Et3N, crown ethers, and tris-(3,6 dioxaheptyl)amine to Novozym SP 435-catalyzed transesterification of 1-azido-3-phenoxy-2-propanol has also been reported to enhance enantioselectivity and reaction rates [33]. Kinetic experiments may be used for revealing the type of inhibition in enzymes. By inserting experimental data to the inverted Michaelis–Menten equation this gives straight-line plots (Lineweaver–Burk), which can be extrapolated to yield the characterizing constants of the enzyme. However, the Michaelis–Menten model cannot account properly for the kinetic properties of allosteric enzymes [34]. Kinetic studies of the inhibitory effect of 1-butanol in different solvents on CALB have been performed and the competitive inhibition constant Ki values obtained correlated with the calculated activity coefficients of the substrate, suggesting that desolvation of the alcohol was the changing condition [35].


7.2.6 The E-value of CALB is Influenced by R-Alcohols

When the butanoic ester of 1-phenoxy-4-methoxy-2-butanol was hydrolyzed by CALB catalysis it was discovered that the E-value changed during the conversion [12]. This effect seems to be quite general for a range of secondary esters. Moreover, when the corresponding alcohols (see Scheme 7.1) were esterified with vinyl butanoate catalyzed by CALB, the E-values also changed. During the conversion of the substrate the E-value increased when the reaction was hydrolysis and decreased during a transesterification reaction. The representative analyses from the reactions with 1-phenoxy-2-butanol (1a) and the corresponding butanoate (1b) catalyzed by Novozym 435 are shown in Figure 7.3. A possible explanation was that the effect was connected to the changing concentration of the enantiomeric esters. However, no effect on E was observed when enantiopure (R)-1b, the faster reacting enantiomer of the butanoate of 1-phenoxy2-butanol, was added to the transesterification reaction of 1-phenoxy-2-pentanol (2a). A similar result was obtained when enantiopure (R)-4b, the slower reacting enantiomer of the butanoate of 3-chloro-1-phenoxy-2-propanol, was added to the transesterification reaction of 1-phenoxy-2-butanol (1a). It was concluded that the effect of the changing E-value was not connected to the changing concentration of the ester. In order to verify this 3-chloro-1-phenoxy-2-propanol (4a) was esterified with vinyl butanoate with addition of an enantiopure (R)-alcohol such as (R)-1a, (R)-2a, (R)-5a, (R)-6a, and (R)-7a at 30% conversion (see Figure 7.4 and Scheme 7.2). The reason for adding a different but structurally related (R)-alcohol was that the analysis and E-value calculations would be influenced by the added (R)-alcohol of the actual substrate. In the esterification of 4a a decrease in E was observed

Figure 7.3 The E-values observed during esterification of 1-phenoxy-2-butanol (1a) catalyzed by Novozym 435 (squares) and the hydrolysis of the corresponding butanoate (1b) (circles).

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Figure 7.4 Upper panel: E-values at different degrees of conversion in transesterification reactions with vinyl butanoate of the alcohols 1a, 2a, 3a, and 4a catalyzed by Novozym 435 (this batch is different from the one used in Figure 7.3). Lower panel: E-values at different degrees of conversion in hydrolysis of the corresponding butanoates 1b, 2b, 3b, and 4b catalyzed by Novozym 435.

Scheme 7.2


Figure 7.5 E-values at different degrees of conversion during transesterification of 4a with Novozym 435. The enantiopure (R)-alcohol (R)-1a) was added at 30% conversion and the Evalue increased from 94 to 205 and then decreased again. The effect was also shown for the addition of (R)-5a and (R)-7a: however, there was no effect for (R)-6a.

from 160 at 9% conversion to 94 at 30% conversion. (This decrease in E corresponded to a small difference in ∆∆G# of 1.55 kJ/mol (0.37 kcal/mol).) However, when (R)-1a was added to the transesterification reaction of 4a at 30% conversion the E-value increased from 94 to 205 before it decreased again. Esterifications of 4a with addition of (R)-1-phenoxy-2-hexanol ((R)-5a) and (R)-2-methyl-1,4butanediol ((R)-7a) showed similar results. The addition of 1-phenoxy-2-pentanol ((R)-2a) and (R)-phenyl ethanol to the resolution of 4a showed similar results, however with a smaller increase in E. The addition of (R)-1-methoxy-2-propanol ((R)-6a) surprisingly did not affect the selectivity. (Figure 7.5) It was also surprising that the E-value in the case of (R)-1-phenoxy-2-hexanol ((R)-5a) increased and also dropped to the low level quickly after addition. It has been shown that 5a is esterified by vinyl butanoate and CALB with almost no selectivity and with a low rate [14]. For that reason, it seems unlikely that the disappearance of the selectivity enhancement was only due to esterification of the alcohol. However, it is possible that the enantiopure alcohols after addition were bound to the enzyme and after a while brought into the bulk solution by the solvent. The observed quick decrease of effect after addition may therefore be a combination of the removal of the (R)-alcohol by esterification or by solvation. Since this effect was observed with the immobilized preparation of CALB (Novozym 435) it was interesting to perform the same reactions with pure protein preparations of CALB to see if the immobilization of the enzyme influenced the changing E-value. The formulated CALB Novozym 525 F is a water solution of CALB containing 1–10% pure protein. After freeze drying at −80 °C this dry protein

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Figure 7.6 E-values from the transesterification reaction of 4a with Novozym 435 (4a/435, squares) and with Novozym 525 F (4a/525 F, circles) in hexane with vinyl butanoate as the acyl donor. The E-value decreased during conversion in both reactions.

powder was used for catalyzing the transesterification reactions of 1a, 2a, 3a, and 4a with vinyl butanoate in hexane. The E-values in all of the reactions decreased with increasing conversion. Figure 7.6 shows the E-values of the esterification of 4a with Novozym 525 F (4a/525 F) plotted together with the resolution of 4a with Novozym 435 (4a/435). It was inferred that immobilization did not significantly influence the reaction. Since addition of the enantiopure (R)-alcohols increased the selectivity of CALB, it is likely that they were causing a conformational change in the enzyme, possibly due to an allosteric effect. However, an allosteric center is not mentioned in the structure details for CALB [36, 37]. Saturation transfer difference nuclear magnetic resonance experiments have shown that there is a strong interaction between the enzyme and the (R)enantiomers of the alcohols, but not the (S)-enantiomers (H. W. Anthonsen, unpublished results). As described above, addition of a small amount of pure (R)alcohols increased the E-value of kinetic resolution and, moreover, the effect disappeared quickly. The reason is probably that the alcohol moves into the active site where it is esterified. A future goal is to find an additive that can bind irreversibly to this ‘unknown’ allosteric site, thus causing a lasting effect. 7.2.7 Is a Changing E Caused by the Slow or the Fast Enantiomer?

When enantiomer selectivity in kinetic resolutions changes as the reaction conditions change, as discussed above, it may be due to an increased or decreased

7.3 Asymmetrization of Prochiral Compounds

Scheme 7.3

reaction rate of one or both of the enantiomers. The reaction rate of each of the enantiomers can be determined by calculating the concentration of the two enantiomers at different degrees of conversion from the equations below [38]. cvF = c × (1 + ee p ) cvS = c × (1 − ee p ) cvF and cvS are the conversions of the fast- and slow-reacting enantiomers (% converted of total concentration of substrate enantiomer), respectively, c is the conversion of the racemic substrate and eep is the enantiomeric excess of the product. The results can be plotted in Kaleidagraph 3.0 and the progress curves of different reactions are compared. The effect of increasing the small substituent of 1a by one CH2 unit as in 2a is shown in Figure 7.7. It is evident that the reason for the decrease in E is due to a slower reaction of the faster enantiomer [14]. This is the opposite of the effect observed when the ethyl ester of 8a is changed into tert-butyl as in 9a (Scheme 7.3). In this case the higher E-value of 9a is due to a lowering of the rate of the slower enantiomer [39]. For 1a and 2a the slow enantiomers are almost equally slow, while for 8a and 9a the fast enantiomers are equally fast.

7.3 Asymmetrization of Prochiral Compounds 7.3.1 Asymmetrization of Prochiral Dicarboxylates: Single-Step Process

Asymmetrization of a prochiral dicarboxylic acid diester catalyzed by lipases, where the stereo center of the product is located on the acyl side, becomes a singlestep process because the polar carboxylic acid and/or amide formed are not well accepted as substrates by the lipase. One example is the enantioselective hydrolysis or ammonolysis of diethyl 3-hydroxyglutarate, as shown in Scheme 7.4, a reaction which leads to the formation of a precursor for the important chiral side chain of atorvastatin, lipitor [40, 41]. The S-enantiomer was formed with high e.e. (98%), but unfortunately this is the undesired enantiomer for the production of the pharmaceutically important product. Only α-chymotrypsin gave a predominance of the

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Figure 7.7 The upper panel shows the progress of esterification of the two enantiomers of 1-phenoxy-2-butanol (1a) catalyzed by Novozym 435. The graph labelled (R)-1a represents the product formed from (R)-1a. For comparison, the corresponding

Scheme 7.4

course of esterification of the two enantiomers of 1-phenoxy-2-pentanol (2a) is also shown. The lower panel shows a comparison between the esterifications of ethyl 4-chloro-3-hydroxybutanoate (8a) and the corresponding tert-butyl ester 9a.

7.3 Asymmetrization of Prochiral Compounds

Scheme 7.5

R-enantiomer, but with a modest e.e. In such a single-step process the e.e. depends only on the ratio k1/k2 which is constant throughout the reaction since the starting material is achiral and since none of the products should give rise to allosteric effects. 7.3.2 Asymmetrization of Prochiral Diol: Double-Step Process

A prochiral diol can be converted into enantiomeric compounds as shown by the example in Scheme 7.5. Catalyzed by a hydrolytic enzyme like a lipase, it can be enantioselectively acetylated by vinyl acetate in organic solvent to yield a mixture of monoacetates. The chiral monoacetates will be formed at unequal rates and also react further at unequal rates. It is usually anticipated that if k1 > k2, then k4 > k3 and, moreover, that the ratio k1/k2 is constant throughout the reaction [4c]. Thus the e.e. of the monoacetate stage will increase due to both steps. When these anticipations apply, the e.e. of the monoacetates should increase throughout the reaction to reach 100% and, moreover, the yield of the two monoacetates (R + S) should increase before it eventually declines and reaches zero when the conversion has reached 100% and only the achiral diacetate will comprise the reaction mixture [4d] (Figure 7.8). 7.3.3 Is the e.e. Constant During Asymmetrization Reactions?

Recent studies using the primary diol shown in Scheme 7.5 and porcine pancreatic lipase (crude, Sigma Type II) as transesterification catalyst, indicate that the situation is more complex [42]. In one reaction, 60% of the diol was converted to enantiomer A of the monoacetate and the e.e. reached 90% after 30 min of conversion. Then enantiomer B was formed resulting in a drop of the e.e. to zero. During further reaction enantiomer B was in excess and the e.e. increased again, but then in favor of enantiomer B. Moreover, after 20 h of conversion, all of the diol was consumed and, surprisingly, the diacetate was virtually absent from the reaction mixture. These results indicate that it is not only important to monitor the course

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Figure 7.8 The graph shows how the e.e. of the mono-ester stage of a double-step process changes during conversion (upper trace) and the sum (%) of mono-esters present (lower trace) at various degrees of conversion.

of the reaction in resolutions, but also during asymmetrization. The unexpected results remain to be explained. They may be due to an allosteric effect as for resolution under 2.5. However, since the enzyme preparation used was not pure, the results may be due to the action of more than one enzyme. Moreover, the possibility of acyl migration should also be considered.

7.4 Conclusions

This chapter has dealt with the production of enantiopure compounds using hydrolases as catalysts. Two kinds of stereo discriminating reactions are discussed: enantiomer selective kinetic resolution and enantioselective asymmetrization of prochiral molecules. The crucial parameter for kinetic resolution is the enantiomeric ratio, the Evalue that is the ratio of the rate constants for reaction with the two enantiomers. A high E-value ensures high enantiomeric excess of the products. The success of asymmetrization depends on a high value for the ratio k1/k2, k1 being the rate constant leading to the formation of one enantiomer and k2 leading to the opposite. It is generally believed that both the E-value and the k1/k2 ratio does not change during the reaction: however, it has recently been shown that this is not always true. When the stereo center of a racemic ester is located in the alkyl part, liberation of the R-alcohol during hydrolysis leads to an increase in the E-value. On the other hand, removal of the R-alcohol during transesterification produces a decrease in E. This is due to an allosteric effect caused by the presence of R-alcohols. Asymmetrization of prochiral diols by esterification also shows anomalous behavior. More experiments have to be carried out before final conclusions can be drawn.

References

Asymmetrization of prochiral molecules by hydrolysis when the stereo center is located on the acyl side is more straightforward, probably because the chiral monocarboxyl products are usually no substrates for a lipase. The general rule seems to be that the E-value changes during resolution and that the e.e.-value depends on the degree of conversion during asymmetization. Principally this should not be restricted to enzymatic catalysis, but also be valid for reactions catalyzed by non-enzymatic catalysts.

References 1 Chen, C.-S., Fujimoto, Y., Girdaukas, G. and Sih, C.J. (1982) Journal of the American Chemical Society, 104, 7294–7299. 2 Anthonsen, T. and Jongejan, J.A. (1997) Methods in Enzymology, 286, 473–495. 3 IUPAC (1996) Basic Terminology of Stereochemistry, http://www.chem.qmul. ac.uk/iupac/stereo/ (accessed October 2nd, 2008). 4 (a) Faber, K. (2004) Biotransformations in Organic Chemistry, 5th edn, SpringerVerlag, Berlin, p. 39. (b) Faber, K. (2004) Biotransformations in Organic Chemistry, 5th edn, SpringerVerlag, Berlin, p. 348. (c) Faber, K. (2004) Biotransformations in Organic Chemistry, 5th edn, SpringerVerlag, Berlin, p. 35. (d) Faber, K. (2004) Biotransformations in Organic Chemistry, 5th edn, SpringerVerlag, Berlin, p. 36. 5 Fersht, A. (1999) Structure and Mechanism in Protein Science, W.H. Freeman and Co., New York. 6 Anthonsen, T. (2001) Basic Biotechnology, 2nd edn (eds B. Kristiansen and C. Ratledge), Cambridge University Press, Cambridge, pp. 409–428. 7 Kazlauskas, R.J., Weissfloch, A.N.E., Rappaport, A.T. and Cuccia, L.A. (1991) The Journal of Organic Chemistry, 56, 2656–2665. 8 Hoff, B.H. and Anthonsen, T. (1999) Chirality, 11, 760–767. 9 Sih, C.J. and Wu, S.-H. (1990) Topics in Stereochemistry. 19, 63–125. 10 Anthonsen, H.W., Hoff, B.H. and Anthonsen, T. (1996) Tetrahedron: Asymmetry, 7, 2633–2638.

11 Anthonsen, H.W. (1996–1997) http://www. chem.ntnu.no (accessed October 2nd, 2008). 12 Waagen, V., Partali, V., Hansen, T.V. and Anthonsen, T. (1994) Protein Engineering, 7, 589–591. 13 Hoff, B.H., Anthonsen, H.W., Anthonsen, T. and T. (1996) Tetrahedron: Asymmetry, 7, 3187–3192. 14 Jacobsen, E.E., Hoff, B.H. and Anthonsen, T. (2000) Chirality, 12, 654–659. 15 Jacobsen, E.E., van Hellemond, E.W., Moen, A.R., Prado, L.C.V. and Anthonsen, T. (2003) Tetrahedron Letters, 44, 8453-8455. 16 Kvittingen, L., Sjursnes, B., Anthonsen, T. and Halling, P. (1992) Tetrahedron, 48, 2793–2802. 17 Jacobsen, E.E., Anthonsen, T. and C. (2002) Journal of Chemistry – Revue Canadienne de Chimie, 80, 577–581. 18 Hartmeier, W. (1986) Immobilisierte Biokatalysatoren, Springer-Verlag, Berlin. 19 Heinsman, N.W.J.T., Schröen, C.G.P.H., van der Padt, A., Franssen, M.C.R., Boom, R.M. and van’t Riet, K. (2003) Tetrahedron: Asymmetry, 14, 2699–2704. 20 Jacobsen, E.E., Andresen, L.S. and Anthonsen, T. (2005) Tetrahedron: Asymmetry, 16, 847–850. 21 Bousquet-Dubouch, M.-P., Graber, M., Sousa, N., Lamare, S. and Legoy, M.-D. (2001) Biochimica et Biophysica Acta, 1550, 90–99. 22 Patkar, S.A., Björkling, F., Zundel, M., Schulein, M., Svendson, A., Heldt-Hansen, H.P. and Gormsen, E. (1993) Indian Journal of Chemistry, 32B, 76–80. 23 Guo, Z.-W. and Sih, C.J. (1989) Journal of the American Chemical Society, 111 (17), 6836–6841.

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7 Factors Affecting Enantioselectivity: Allosteric Effects 24 Itoh, T., Ohira, E., Takagi, Y., Nishiyama, S. and Nakamura, K. (1991) Bulletin of the Chemical Society of Japan, 64, 624–627. 25 Homann, M.J., Vail, R., Morgan, B., Sabesan, V., Levy, C., Dodds, D.R. and Zaks, A. (2001) Advanced Synthesis Catalysis, 343, 744–749. 26 Lundhaug, K., Overbeeke, P., Jongejan, J. and Anthonsen, T. (1998) Tetrahedron: Asymmetry, 9, 2851–2856. 27 Akeboshi, T., Ohtsuka, Y., Ishihara, T. and Sugai, T. (2001) Advanced Synthesis Catalysis, 343, 624–637. 28 Bamann, E. and Laeverenz, P. (1930) Hoppe-Seyler’s Zeitschrift fur Physiologische Chemie, 193, 201–214. 29 Ammon, R. and Fischgold, H. (1931) Biochemische Zeitschrift, 234, 54. 30 Berger, B., Rabiller, C.G., Königsberger, K., Faber, K. and Griengl, H. (1990) Tetrahedron: Asymmetry, 1, 541–546. 31 Maugard, T., Remaud-Simeon, M., Petre, D. and Monsan, P. (1997) Tetrahedron, 14, 5185–5194. 32 Parker, M.-C., Brown, S.A., Robertson, L. and Turner, N.J. (1998) Chemical Communications, 2247–2248.

33 Pchelka, B.K., Loupy, A., Plenkiewicz, J. and Blanco, P.A.L. (2001) Tetrahedron: Asymmetry, 12, 2109–2119. 34 Berg, J.M., Tymoczko, J.L. and Stryer, L. (2002) Biochemistry, W.H.Freeman, New York. 35 Garcia-Alles, L.F. and Gotor, V. (1998) Biotechnology and Bioengineering, 59, 163-170. 36 Uppenberg, J., Hansen, M., Patkar, S. and Jones, T.A. (1994) Structure, 2, 293–308. 37 Uppenberg, J., Öhrner, N., Norin, M., Hult, K., Kleywegt, G.J., Patkar, S., Waagen, V., Anthonsen, T. and Jones, T.A. (1995) Biochemistr y, 34, 16838–16851. 38 Hoff, B.H. (1999) Ph.D. Thesis, NTNU, 78. 39 Hoff, B.H., Anthonsen, T. and (1999) Tetrahedron: Asymmetry, 10, 1401–1412. 40 Jacobsen, E.E., Hoff, B.H., Riise Moen, A. and Anthonsen, T. (2003) Journal of Molecular Catalysis. B: Enzymatic, 21, 55–58. 41 Moen, A. Riise, Hoff, B.H., Hansen, L.K., Anthonsen, T. and Jacobsen, E.E. (2004) Tetrahedron: Asymmetry, 15, 1551–1554. 42 Lie, A., Ljones, T., Hoff, B.H., Anthonsen, T. and Jacobsen, E.E. (2008) to be published.

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8 Kinetic Resolution of Sec-alcohol in Non-conventional Media Maja Habulin, Mateja Primožicˇ and Željko Knez

8.1 Introduction

The regioselectivity of enzymes, even on complex molecules, without any need for protecting groups is a fundamental strength of biocatalysis. Therefore with biocatalysis as a tool for chemical transformations performance the number of synthetic steps and thereby the occupation time of chemical reactors, which is an important factor of process economy, could be reduced. The potential utility of biocatalysis in the pharmaceutical industry has been widely recognized in the last decade. Considering the high catalytic power and exceptional stereo-, regio-, and chemoselectivity, the area of biotransformations in the pharmaceutical industry is now experiencing a significant growth due to the wide range of commercial applications [1]. Over the past decade the view of organic chemists has also changed with regard to the use of enzymes in organic synthesis, from downright ignorance to careful acceptance of this technology [2]. Green chemistry is searching for alternative, environmentally friendly reaction media as compared to traditional organic solvents. The benefits are an increase in reaction rates and lower reaction temperatures, as well as higher selectivities. Most chemical processes depend on solvents. The use of volatile organic solvents in chemical processes has a detrimental impact on the environment and human health, emphasizing the need for alternative clean manufacturing processes and environmentally benign technologies. Some of these solvents are miscible with water and therefore they must be stripped from water before it leaves the process, not only for ecological but also for economic reasons. Solvents must be recovered for recycling and reuse for an economically viable process. Ionic liquids (ILs) and supercritical fluids (SCFs) are alternative solvents that may also be used in green chemistry [3]. ILs are low melting point salts that represent an exciting new class of reaction solvents for catalysis. ILs are composed entirely from ions and possess negligible vapour pressures and, with proper selection of the wide range of possible cations and anions, the solvent properties of ILs could be controlled. ILs have been accepted as a new green chemical revolution, which has excited both academia Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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8 Kinetic Resolution of Sec-alcohol in Non-conventional Media

and chemical industries. This new chemical group can reduce the use of hazardous and polluting organic solvents due to their unique characteristics as well as taking part in various new syntheses [3]. One of the most exciting recent developments in the use of ILs for catalytic systems is the application of biocatalysis in these solvents [4] due to the good stability of enzymes in ILs. SCFs, as solvents, show the most attractive properties, as they have gas-like low viscosities and high diffusivities and, simultaneously, they possess attractive liquid-like solubilizing power. Supercritical carbon dioxide (SC-CO2) is by far the most studied SCF, because of its economical, technical, environmental, and health advantages. Moreover, it is an excellent solvent for the transport of hydrophobic compounds. Furthermore, its critical pressure is relatively low (7.34 MPa) and it has an ambient critical temperature (304.45 K), which provides the mild conditions necessary for keeping its enzyme activity. It has GRAS (generally regarded as safe) status and can therefore be used as an alternative for organic solvents. One of the main advantages of using SC-CO2 as a reaction medium lies in its fractionation potential. It exhibits a wide spectrum of solvent properties, due to the continuous variation of its density versus pressure and temperature. In recent years, among non-conventional bioreaction media, the SCF/IL biphasic system has increasingly attracted attention as an emerging green and hightechnology alternative to classical organic solvents for carrying out enzymatic reactions for the preparation of valuable and active materials [5]. Clean chemical processes, which provide directly pure products by using appropriate enzyme/IL systems in combination with SCFs, contain an exciting potential for developing the upcoming green chemical industry. The volatile and non-polar SC-CO2 forms different two-phase systems with nonvolatile and polar ILs. The product recovery process with these systems is based on the principle that SC-CO2 is soluble in ILs, but ILs are not soluble in SC-CO2 [6]. Since most organic compounds are soluble in SC-CO2, these products are transferred from the IL to the supercritical phase [7, 8]. Enzyme-catalyzed reactions based on such biphasic systems have been shown to be promising alternatives for developing green chemical processes because of their physical and chemical characteristics [9]. By combining these media with enzymes, the possibilities of carrying out integral green biocatalytic processes has been already demonstrated [10–12]. Such biphasic systems can be used for both the biotransformation and extraction of products simultaneously, even under extremely harsh conditions, because of the different miscibilities of ILs and SC-CO2. Chiral alcohols are useful starting materials for the synthesis of various biologically active compounds. The need for enantiomerically pure drugs and agrochemicals has increased in recent years [13]. Derivatives of enantiopure 1-phenylethanol are important chiral building blocks, which can be used as synthetic intermediates for the production of pharmaceuticals, fine-chemicals agrochemicals, and natural products. In particular (R)-1-phenylethanol is in widespread use as an ophthalmic preservative, an inhibitor of cholesterol intestinal adsorption, a solvatochromic dye, a fragrance, and so on.

8.2 SCFs – Replacement for Organic Solvents in Biocatalysis

Lipase B from Candida antarctica (CALB) has been shown to be an excellent enantioselective biocatalyst for the stereo-selective acylation of racemic alcohols [14, 15]. The most often used commercial preparation of CALB is Novozym 435, where the enzyme is immobilized on a macroporous acrylic resin and the matrix presents about 90% of the total mass. In this research the kinetic resolution of 1-phenylethanol catalyzed by commercially available immobilized lipase from CALB was assayed in non-aqueous conditions in SC-CO2 and IL/SC-CO2 systems with the aim of studying the enantioselectivity of Novozym 435. The influence of different reaction parameters, such as pressure, the acyl donor/alcohol molar ratio and different ILs, on the enantiomerically pure compound (R)-1-phenylethyl acetate formation via kinetic resolution of 1-phenylethanol was investigated.

8.2 SCFs – Replacement for Organic Solvents in Biocatalysis

The breakthrough of biocatalysis in non-aqueous media started in the early 1980s [16–19] when enzyme-catalyzed reactions (e.g. esterification and transesterification) began to be performed in organic solvents (e.g. hexane and heptane). Changing the hydrophobicity of predominantly aqueous media through the addition of organic solvents causes the hydrophobic effects, which keep the hydrophobic residues buried in the core of the proteins and folded in an aqueous environment where enzymes are kinetically trapped in their native structure in organic solvents [20–23]. Enzymes are frequently more stable in organic solvents than in water. The use of organic solvents for food and health products is nowadays being progressively restricted. Therefore, attempts have been made to use SCFs as alternative solvents [24, 25]. They could, like organic solvents, stabilize the enzymes and dissolve the hydrophobic compounds and are able to shift the thermodynamic equilibrium of reaction toward synthesis. Moreover, SCFs possess gas-like diffusivities and low viscosities, which leads to a reduction in the mass transfer resistance between the reaction mixture and the active sites of the enzyme, resulting in increased reaction rates. In addition, the dielectric constant and density of an SCF are very sensitive to pressure and temperature. Consecutively the reaction environment and, thus, the activity and specificity of enzymes could be controlled by changing the pressure and temperature. Another advantage of using SCFs as a solvent for enzyme-catalyzed reactions is simpler downstream processing in comparison with those in liquid solvents. Since SCFs are usually gases at ambient conditions, the solvent can be easily removed without leaving any residues in the product required for industrial manufacture, cosmetic and pharmaceutical applications. Among the solvents that can be used in supercritical conditions for conducting biocatalytic reactions, the most studied one has been CO2, because of its low critical temperature. Apart from being non-toxic, easily available, and cheap, many enzymes have proven to be stable and active in it [26]. Enzymes as biocatalysts require narrow operation parameters. Elevated pressures and temperatures,

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as well as an extreme pH, may lead to deactivation of the protein. In SCFs there are direct effects of pressure on the enzyme activity, which may lead to denaturation and indirect effects of pressure on the enzymatic activity. In the case of SCCO2 only small direct effects of pressure with regard to enzyme inactivation are expected. The protein structure should retain on the whole and only local changes may occur. Those local changes may lead to another active state of a protein, which may posses an altered activity, specificity, and stability. Pressure is also likely to affect the reaction indirectly by changing either the rate constant or the reactants solubility. At higher pressures more solute–solvent interactions take place, resulting in a better solvent capacity. Therefore, the pressure effect on the enantioselectivity of commercially available immobilized CALB (Novozym 435) for the kinetic resolution of 1-phenylethanol was studied in non-aqueous SC-CO2.

8.3 Effect of Pressure

An enzyme stability test in SC-CO2 was performed at different pressures. CALB was incubated overnight at certain conditions and afterwards its residual activity was determined. No direct influence of the pressure on its biocatalyst activity was found. When performing the resolution of 1-phenylethanol, the conversion dependence on the pressure was noticed at the near-critical region when the reaction was performed in almost non-aqueous SC-CO2. The pressure was varied from 6.7 to 19 MPa. At a temperature of 313.15 K the yield in the desired enantiopure compound ((R)-1-phenlyethyl acetate) rose when the pressure was increased up to 9 MPa due to the solubility change with pressure near the critical point. The phase behavior of the system also plays an important role in the reaction. The ratelimiting step of (R)-1-phenlyethyl acetate formation in SC-CO2 was the diffusion of substrates from the reaction bulk through the boundary layer to the external surface of the enzyme support. Because of the enhanced mass transfer the reaction rate increased in the pressure range between 6.7 and 9 MPa, which can be observed from Figure 8.1. Visual observations of the phase behavior at different reaction conditions are presented in Figure 8.2. The reaction was carried out in a liquid reaction bulk in the contact with the solid biocatalyst at 9 MPa. At a pressure close to 11 MPa at the chosen temperature and composition conditions the supercritical phase was attained for the whole reaction bulk. On the basis of the biocatalyst stability results it was obvious that there were indirect effects that influenced the reaction, for example the density-dependent physical properties were affected by the change in pressure. An increase in pressure led to an enhanced solvent density with improvement in its solvating power in the reaction bulk. The solubility in the liquid reaction mixture increased with pressure as well. In the liquid subcritical region rich in CO2, at pressures below

8.3 Effect of Pressure

Figure 8.1 Pressure effect on the kinetic resolution of 1-phenylethanol, catalyzed by immobilized CALB in SC-CO2 after five hours of reaction: bars – dependence of the conversion on the pressure; spots – pressure dependence of the enantiomeric excess for 1-phenylethanol.

Figure 8.2 Visual vapour-liquid phase equilibrium observations of the reaction bulk: liquid reaction bulk in contact with the solid biocatalyst up to 6 MPa (left panel); liquid reaction bulk in contact with the solid biocatalyst up to 9 MPa; (middle panel); supercritical reaction bulk in contact with the solid catalyst upon 15 MPa (right panel).

9 MPa the diffusivity increased significantly and the surface tension and viscosity were reduced. Kinetic observations on the pressure effect showed that that liquid reaction mixture rich in SC-CO2 led to higher performance than when diluted in a single supercritical phase. The highest conversion was obtained at 9 MPa. At pressures above 9 MPa the conversion slightly decreased: with a larger amount of CO2 at higher pressures the dilution effect of the substrates appeared and the reaction was slowed down. At 15 MPa only one phase was present. In this case the higher density was counter-balanced by the substrates’ mole fraction decrease in the enzyme phase.

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Figure 8.3 Influence of acyl donor/alcohol molar ratio on the kinetic resolution of 1-phenylethanol, catalyzed by immobilized CALB in SC-CO2 after 5 hours of reaction: bars – dependence of the conversion on the pressure; spots – dependence of the enantiomeric excess on the pressure.

8.4 Effect of the Acyl Donor/Alcohol Molar Ratio

For a reversible reaction an increase in the acyl donor concentration results in higher product yields. In this case the chemical equilibrium is shifted towards synthesis. On the other hand, high concentrations of substrates may cause inhibition and the reaction is slowed down. For (R)-1-phenylethyl acetate formation the effect of the substrate vinyl acetate/1-phenylethanol molar ratio on the final conversion was studied. The results are presented on Figure 8.3. A higher yield of the enantiopure compound was achieved when raising the acyl donor molar concentration with respect to the alcohol concentration. A conversion of 49.9% was obtained at an acyl donor/alcohol molar ratio of 9/1. After 5 h of reaction at tested conditions a complete conversion of (R)-1-phenylethanol into the enantiopure (R)-1-phenylethyl acetate was attained. The enantiomeric excess for reactants (eeR) was 99.9%.

8.5 ILs – Solvents for Sustainable Technology in Biocatalysis

ILs represent a new class of salts that are distinguished by a range of useful properties such as negligible vapor pressure, thermal stability, non-flammability, a high ionic conductivity, and remarkable solubility properties and have rapidly found a

8.5 ILs – Solvents for Sustainable Technology in Biocatalysis

place of choice as valuable environmentally benign media substitutes for many volatile solvents [8, 27–30]. By modification of the cation and of the anion, their properties may be widely changed. The commonly used cations for IL synthesis are based on imidazolium, ammonium, phosphonium, pyridinium, pyrrolidinium, and so on. ILs anions are either organic [RCO2]− or inorganic and can be classified in two groups: polynuclear − ( Al 2Cl 7− , Al3Cl10 , Au 2Cl 7− , Fe2Cl 7− …) and mononuclear anions which lead to neutral − stoichiometric ILs (BF4− , PF6−, SbF6−, N(CF3SO2 )2 …) [31–33]. The properties and behavior of the IL can be adjusted separately for each individual reaction and, therefore, they can be described as designer solvents. By choosing the correct IL, high product yields can be obtained and a reduced amount of waste can be produced in a given reaction. Often the IL can be recycled and this leads to a reduction in the costs of the processes. Reactions in ILs are not difficult to perform and usually require no special apparatus or methodologies. The reactions are often quicker and easier to carry out than in conventional organic solvents. Several thermodynamic and kinetic behaviors of enzyme-catalyzed reactions performed in ILs, with respect to enzymatic reactions carried out in conventional solvents, could lead to an improvement in the process performance [34–37]. ILs showed an over-stabilization effect on biocatalysts [38] on the basis of the double role played by these neoteric solvents: ILs could provide an adequate microenvironment for the catalytic action of the enzyme (mass transfer phenomena and active catalytic conformation) and if they act as a solvent, ILs may be regarded as liquid immobilization supports, since multipoint enzyme-IL interactions (hydrogen, Van der Waals, ionic, etc.) may occur, resulting in a flexible supramolecular not able to maintain the active protein conformation [39]. Their polar and non-coordinating properties hold considerable potential for enantioselective reactions since profound effects on reactivities and selectivities are expected [40]. In recent years attention has been focused on the application of ILs as reaction media for enantioselective processes [41–43]. Extraction of the product from the IL in pure form can pose a problem. Water soluble compounds can be easily extracted with water. For separation of compounds with high vapor pressure the distillation could be used and higher temperatures would be required in order to extract products with low vapor pressures, which could result in decomposition of the product. A great number of researchers have investigated the high-pressure phase behavior of IL/CO2 systems and found that CO2 is highly soluble in most ILs and that ILs are not measurably soluble in SC-CO2. Volatile and non-polar SC-CO2 together with non-volatile and polar ILs forms a new system with unique properties that have been utilized for extracting organic compounds from ILs using SC-CO2 [3]. The influence of different ILs, based on N, N′dialkylimidazolium cations as reaction media, on the enantioselectivity of commercially available immobilized lipase from CALB for the kinetic resolution of 1-phenylethanol was studied. Further, the performance of the enzymatic kinetic 1-phenylethanol resolution by CALB in the system [bmim][PF6]/SC-CO2 was studied.

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8.6 ILs, Based on the N, N′-Dialkylimidazolium Cations as Reaction Media

The use of ILs contributes many advantages in biocatalysis, including enhanced stability, activity, and stereoselectivity [41, 44, 45]. When combined with SC-CO2, ILs showed a promising media for enhanced stereoselectivity and stability of biocatalysts [9, 46–48]. The use of SC-CO2 and room temperature ILs as a combined bioreaction media has been widely demonstrated [5, 7, 8]. There has been much interest in research on an efficient enzymatic system in IL/CO2 media for racemates kinetic resolution [49, 50]. Due to the wide spectrum of their physicochemical properties, several ILs, based on the N, N′-dialkylimidazolium cations were used for testing the enantioselectivity of CALB [34–36]. To screen different ILs for the kinetic resolution of 1-phenylethanol, ILs associated with mononuclear anions, such as tetrafluoroborate [ BF4− ], hexafluorophosphate [ PF6− ], and triflimide [ NTf 2− ], based on dialkylimidazolium cations, were used as reaction media at atmospheric pressure and 313.15 K. All the ILs examined were found to be proper media for the biocatalytical reaction system studied in terms of enantioselectivity. The inclusion of the CALB in the IL matrix may provide the protein with a conformation suited to its enantioselective action. The highest conversion was achieved in [bmim][ PF6 ] and in [emim][ NTf2 ] a slow enzyme deactivation was observed, as can be observed from Figure 8.4. Low hydrogen bond basicity minimizes interference with the internal hydrogen bonds of the enzyme [51]. In all three ILs, [ BF4− ], [ PF6− ], and [ NTf 2− ], the anions have low hydrogen bond basicity, as the [ BF4− ] anion spreads its negative charge over four fluorine

Figure 8.4 Three different ILs; [bmim][PF6], [bmim][BF4] and [emim][NTf2] as reaction media for the enzymatic kinetic resolution of 1-phenylethanol by immobilized CALB after five and 24 hours of reaction.

8.8 The [bmim] [PF6]/SC-CO2 System as a Reaction Medium

atoms, the [NTf2−] anion over five atoms, and the [PF6−] anion over six fluorine atoms and great activity of CALB was detected. The different properties of ILs, with regard to their polarity, hydrophobicity, and solvent miscibility behavior through combination with different anions, are the reason for the different biocatalyst activities. Good to excellent activity of CALB was observed with a decrease in polarity and hydrophobicity and a viscosity increase of the ILs. In [bmim][PF6] a conversion of (R)-1-phenylethanol into the ester of 48.9% and an ee of 95.6% were achieved after 5 h and 100% of (R)-1phenylethanol was converted into the enantiopure (R)-1-phenylethyl acetate after a 1-day reaction. Immobilized CALB exhibited excellent stability, activity, and selectivity towards the (R)-enantiomer of 1-phenylethanol in [bmim][PF6]. In some research bis(trifluoromethylsulfonyl)imide-based ILs have been regarded as very suitable media for biocatalysis [39, 46, 50]. On the contrary, in the present work, lower suitability of the same IL was demonstrated. Since immobilized CALB catalyzed both hydrolytic and transesterification reactions, its enantioselectivity for long reaction times was lower.

8.7 ILs/SCFs Biphasic Systems as Promising Media for Biocatalysis

Enzyme transformations in ILS/SC-CO2 biphasic systems, whereby enzyme molecules are ‘immobilized’ in the IL phase and substrates/products are transported by the SC-CO2 phase, are described as a way for carrying out clean synthetic chemical processes to produce pure products [52].

8.8 The [bmim][PF6]/SC-CO2 System as a Reaction Medium

Kinetic resolution of 1-phenylethanol catalyzed by CALB was carried out in the IL/SC-CO2 biphasic system. To prevent undesirable reactions and ensure better conversion of (R)-1-phenylethanol, [bmim][PF6] was chosen for this kind of experiments. Because of the possible direct and indirect effects of the pressure on the activity of biocatalyst its influence was studied between 6 and 36.5 MPa. At all conditions examined a biphasic reaction medium was attained, which is illustrated in Figure 8.5. The enzyme was suspended in the IL phase, where the reaction took place. The substrates and products resided largely in the supercritical phase, which was also the extractive phase. A strong influence of the pressure on the reaction was registered at pressures lower than 16 MPa, where an important increase in the CO2 molar fraction in the liquid phase appeared. The phase behavior of CO2/[bmim][PF6] showed that the solubility of SC-CO2 in this IL markedly increased up to 16 MPa, where up to 65% mol/mol of CO2 was registered in the liquid phase. At 16 MPa the highest conversion of (R)-1-phenylethanol into the enantiopure (R)-1-phenylethyl acetate was the

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Figure 8.5 Visual vapour-liquid phase equilibrium observations of the reaction bulk, where liquid reaction bulk was in the contact with the solid biocatalyst and the upper phase was saturated with substrates and products, extracted by SC-CO2.

Figure 8.6 Performance of the enzymatic kinetic 1-phenylethanol resolution over immobilized CALB in [bmim][PF6]/SC-CO2 system at 313.15 K by varying the pressure: bars – dependence of the conversion on the pressure; spots – dependence of the enantiomeric excess on the pressure.

highest, approx. 46.8%, resulting in enantiomeric excess of 88.1%. From Figure 8.6 it may be observed that at pressures above 16 MPa no strong influence on the biocatalyst activity was detected. Biocatalyst particle shape and size characterization before and after treatment in the two selected media was examined in order to ascertain the role of the solvent on enzyme structure. The crude biocatalyst preparation consisted of spherical micro-granules with a mean particle size of about 500 µm. No particle size modifications were observed by SEM when the biocatalyst was treated by SC-CO2. After treatment in [bmim][PF6] a lower mean particle size of about 430 µm was observed, probably due to partial carrier dissolution. Conversely, by exposing the biocatalyst

8.9 Effect of Acyl Donor Concentration

particles to the combination of [bmim][PF6]/SC-CO2 no changes in the particle size were detected. The IL/SC-CO2 media for 1-phenylethanol resolution, catalyzed by CALB, offers a number of advantages, such as high resolution yields, simple downstream processing, guarantying an enzyme overstabilization effect and exceptional enzyme enantioselectivity with no transformation of the not desired enantiomer, albeit apparently lower conversions were registered in this medium compared to the ones, obtained in SC-CO2.

8.9 Effect of Acyl Donor Concentration

The influence of acyl donor concentration in the IL/SC-CO2 medium was exploited in order to achieve higher enantiopure yield (Figure 8.7) at 313.15 K and 16 MPa. The highest conversion was achieved at the same substrate composition as in SC-CO2, namely vinyl acetate/1-phenylethanol molar ratio of 9/1. No (S)-1-phenylethanol conversion was detected at all tested conditions, which means an enantiomeric excess for products evaluated higher than 99.9%. Optimal conversion, achieved after five hours of reaction, was 47.2% and enantiomeric excess for reactants was 89.5%. The conversion could be maximized with higher amount of biocatalyst in the reaction mixture. As expected, after five hours of bioconversion, approximately complete conversion 49.9% of (R)-1-phenylethanol into the enantiopure (R)-1-phenylethyl acetate was achieved. Enantiomeric excess for reactants was 99.3%.

Figure 8.7 Acyl donor/alcohol molar ratio effect on the reaction of the enzymatic kinetic 1-phenylethanol resolution by CALB in [bmim][PF6]/SC-CO2.

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8.10 Conclusion

Kinetic resolution of racemic 1-phenylethanol was successfully performed by CALB as the enantioselective biocatalyst using vinyl acetate as the acyl donor. High yield, enantioselectivity and simple downstream processing make SC-CO2 and IL/SC-CO2 systems excellent media for the above mentioned kinetic resolution of sec-alcohol. Pressure effects the reaction to a certain extent, which was explained by exploiting thermodynamic arguments. Besides, the yield may be improved by increasing the acyl donor concentration. The outlook of syntheses, based on using SC-CO2 or appropriate IL/SC-CO2 biphasic system for developing integral green chemical processes due to the physical and chemical characteristics of these neoteric solvents and the enhanced enzyme catalytic properties seems promising.

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9 Strategies for the Biocatalytic Lipophilization of Phenolic Antioxidants Maria H. Katsoura, Eleni Theodosiou, Haralambos Stamatis and Fragiskos N. Kolisis

9.1 Introduction

Natural polyhydroxylated phenolic compounds, including flavonoid and phenolic glucosides as well as flavonolignans, are widely distributed among various plants known for their antioxidant activity and antimicrobial, anticarcinogenic, and antiinflammatory properties [1, 2]. Because of their biological activities, these natural compounds can be used in pharmaceuticals and cosmetics as well as food additives [1]. However, their poor solubility and stability in different media, especially lipidic matrices and lipophilic media (fats, oils and emulsions), limit their application. Moreover, their biological activity seems to depend not only on their chemical structure [3, 4], but also on their degree of lipophilicity, which could enhance their uptake into cells or influence their interaction with proteins and enzymes [5, 6]. The modification of these natural polyhydroxylated compounds via acylation of the hydroxyl functions with aliphatic molecules not only increases their structural diversity, producing analogs that may be useful models for the study of structure–activity relationships, but also changes their physicochemical properties, increasing their solubility in lipophilic media. Moreover, the selective acylation of these natural compounds with various acyl donors could enhance their biological activities, such as their antioxidant and antimicrobial activity, as well as their pharmacological properties [5, 6]. The use of enzymes in non-aqueous organic media for the modification of various substrates, including natural compounds, has been introduced as an advantageous new approach in the last two decades [7, 8]. Several research groups have reported the feasibility of the enzymatic modification of various polyhydroxylated compounds, such as flavonoids, sugars, and glucosides, in both toxic and less toxic organic media using either lipases or proteases [9–12]. However, the use of organic solvents as media for the biocatalytic modification of such polyhydroxylated natural compounds has often shown several disadvantages, such as low yields and limited productivity that arise from the reduced solubility of such compounds in these media [12, 13].


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9 Strategies for the Biocatalytic Lipophilization of Phenolic Antioxidants

Figure 9.1 Structures of natural antioxidants.

On the other hand, several groups have reported the use of ionic liquids (organic salts consisting only of ions, liquid at or near room temperature) as media for the biocatalytic modification of polyhydroxylated compounds, including ascorbic acid [14], sugars, and glucosides [15, 16] as well as flavonoids [5, 17]. Because of their specific physicochemical characteristics, such as lack of vapor pressure, thermal stability, and their ability to dissolve many kinds of compounds, including polar or less polar organic compounds, ionic liquids can be used as alternative green media for various biotransformations [18–20]. It is interesting to note that, toxicological tests of a series of ionic liquids with imidazolium cations and various anions (including PF6− and BF4−) showed that most of the ionic liquids were less toxic than organic solvents by several orders of magnitude [21, 22]. The reduced toxicity of some ionic liquids is an important advantage for their use as media for the biocatalytic modification of compounds that can be used as food additives or in pharmaceutical and cosmetic formulations. The use of various low-water reaction systems, including anhydrous organic solvents as well as imidazolium-based ionic liquids, is investigated for the biocatalytic preparation of lipophilic derivatives of various natural phenolic compounds (Figure 9.1) including phenolic and flavonoid glycosides (esculin, salicin, helicin,

9.3 Results and Discussion

naringin, and rutin) as well as flavonolignan (silybin). The effects of various reaction parameters on the conversion yield and the regioselectivity of the biocatalytic process in different reaction media are discussed.

9.2 Materials and Methods 9.2.1 Materials

Immobilized lipase B from Candida antarctica (CALB) (Novozym 435) was obtained from Novo Nordisk Novozymes. All chemicals and ionic liquids used were of the highest available purity. 9.2.2 Enzymatic Acylation Procedure

The enzymatic acylation of natural polyhydroxylated compounds in ionic liquids ([bmim]BF4 and [bmim]PF6), and organic solvents was carried out in stirred flasks according to the procedure reported elsewhere [5, 6, 17]. 9.2.3 Analytical Methods

Quantitative analysis of reaction mixtures was performed by high-performance liquid chromatography (HPLC) and a diode array ultraviolet detector, as reported elsewhere [5, 6, 17]. 9.2.4 Purification and Chemical Structure Determination of Esters

Highly purified esters were obtained by preparative HPLC [5, 17]. The chemical structure of the purified monoesters was determined by 13C and 1H nuclear magnetic resonance (NMR) and their molecular masses were determined by a mass spectroscopy (MS) technique as reported elsewhere [5, 17].


Herein we discuss the enzymatic modification of natural polyhydroxylated compounds in both organic and ionic liquid media catalyzed by commercially available immobilized CALB.

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9.3.1 Modification of Natural Antioxidants in Organic Solvents 9.3.1.1 Enzymatic Acylation of Rutin and Silybin with Dicarboxylic Acids Silybin, the most biologically active compound of the flavonolignan mixture silymarin, has three phenolic, one primary aliphatic, and one secondary aliphatic hydroxyl group, while rutin has a sugar moiety with only secondary hydroxyl groups. A plethora of studies support that CALB exhibits selectivity towards not only primary OH groups, but towards secondary OH groups as well [5, 23–26]. The lipase preferentially acylates the primary OH group with less steric hindrance and the secondary OH group is acylated next where there is no primary OH available [27]. The regioselectivity of the biocatalyst is affected by the type of the sugar moiety attached to the aglycon flavonoid, in respect to the presence of primary OH groups as well as steric obstructions [5, 12, 28]. Apart from the nature of the polyhydroxylated natural compound, the nature of the acyl donor could influence the performance of the biocatalytic reaction [9, 17]. The acylation of silybin and rutin with three dicarboxylic acids (hexanedioic acid, dodecanedioic acid, and hexadecanedioic acid) is investigated in tert-amyl alcohol. Table 9.1 shows the effect of the nature of the antioxidant and acyl donor on the performance of the reaction. Higher conversion yields were observed for all dicarboxylic acids used in the case of silybin, as compared to rutin, due to the presence of primary hydroxyls in this flavonolignan for which the lipase shows higher specificity. Analysis of the chemical structures of purified products by NMR revealed that the acylation of rutin occurred at the 4″″-OH of the rhamnose moiety, while the aylation of silybin occurred at its unique primary hydroxyl at position C23. It must be noted that, in the case of silybin, both monoester and diester formation was observed and identi-

Table 9.1 Effect of the reaction medium on the conversion yield after 96 h of incubation for the acylation of rutin (7.5 mM) and silybin (7.5 mM for tert-amyl alcohol and tertbutyl alcohol and 30 mM for acetone) with dicarboxylic acids catalyzed by immobilized CALB at 50 °C.

Reaction medium

Conversion % Rutin

tert-Amyl alcohol tert-Butyl alcohol Acetone

Silybin

C6

C12

C16

C6

C12

C16

13.1 6.2 n.d.a

12.3 8.0 n.d.a

10.5 6.4 n.d.a

32.0 17.7 31.3

30.7 13.8 23.3

38.0 22.4 45.5

a Not determined. The substrates were used in equimolar amounts.


fied by MS spectroscopy. Irrespectively of the system used, the oxidized form of silybin (2,3-dehydrosilybin) was also present. When silybin was used as the substrate a higher conversion yield was observed when hexadecanedioic acid was used as the acyl donor, while in the case of rutin no significant effect of the carbon chain length on the conversion yield was observed. 9.3.1.2 Effect of Organic Solvent In order to investigate the effect of organic solvent, tert-amyl alcohol, tert-butyl alcohol, and acetone, were chosen as reaction media, mainly because of their compatibility with the reaction studied, as well as their low toxicity [29]. As can been seen in Table 9.1 the use of tert-butyl alcohol led to lower conversion yields both for rutin and silybin as compared to tert-amyl alcohol. In the case of acetone, higher conversion yields were obtained for C12 and C16 dicarboxylic acids. The solubility capacity of acetone for silybin is about 10 times higher (∼40 g/l) than tert-amyl alcohol and tert-butyl alcohol. The higher solubility of silybin in acetone is advantageous, as the amount of esters obtained was high, reaching values of 4.5, 3.4, and 6.6 g/l for hexanedioic, dodecanedioic, and hexadecanedioic acid, respectively, after 96 h of incubation. 9.3.1.3 Effect of Substrate Concentration In order to investigate the effect of the substrates’ molar ratio on the reaction performance, the concentrations of the enzyme and antioxidant were kept constant, whereas that of the acyl donor was increased. In the case of rutin, independently of the molar ratio, used the acylation proceeded with low yields, while only monoesters were formed in all reaction systems used. The same behavior was observed in the case of silybin using tert-amyl alcohol and tert-butyl alcohol as reaction media (data not shown). In contrast, when acetone was used as medium the conversion yield for the acylation of silybin increased as the molar ratio was increased (Table 9.2).

Table 9.2 Effect of the molar ratio of the acyl donor to

flavonolignan on the conversion yield and reaction rate after 96 h of incubation for the acylation of silybin (30 mM) with dicarboxylic acids catalyzed by immobilized CALB in acetone at 50 °C. Molar ratio

1 2 5

Conversion % (initial reaction rates in mmol/h/g of biocatalyst) C6

C12

C16

31.3 (0.02) 44.5 (0.02) 59.0 (0.04)

23.3 (0.02) 33.2 (0.04) 42.5 (0.04)

45.5 (0.04) 56.5 (0.04) 66.8 (0.04)

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9.3.2 Modification of Natural Antioxidants in Ionic Liquid Media 9.3.2.1 Enzymatic Acylation of Natural Polyhydroxylated Compounds In order to investigate the efficiency and selectivity of the enzymatic acylation of various polyhydroxylated natural compounds in ionic liquid media the transesterification of salicin, helicin, esculin, and naringin, as well as silybin catalyzed by immobilized CALB was investigated using vinyl butyrate as the acyl donor. Two ionic liquids were used, the water-miscible 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), in which the solubility of water is limited. The percentage conversion yield of the enzymatic acylation of the forenamed compounds in ionic liquids as compared to acetone is depicted in Figure 9.2. As can be seen, immobilized CALB efficiently catalyzes the acylation of all polyhydroxylated compounds in the ionic liquid media used, leading to high conversion yields. The reaction rates for the enzymatic acylations are summarized in Table 9.3. Higher reaction rates were obtained for all polyhydroxylated compounds tested when [bmim]BF4 was used as the reaction medium. It is interesting to note that the solubilities of esculin, salicin, helicin, naringin, and silybin at 60 °C were approximately 68, 23, 40, 100, and 82 mM, respectively, in [bmim]BF4. On the other hand, in [bmim]PF6 and acetone, in which the reactions rates were lower, the solubilities of the aforenamed compounds were 20, 5.5, 19.5, 1.5, and 7.2 mM and 6.5,

Figure 9.2 Conversion yields for the enzymatic acylation of different polyhydroxylated natural compounds (30 mM) with vinyl butyrate (300 mM) catalyzed by immobilized CALB in (a) [bmim]BF4 (white bars) and (b) [bmim]PF6 (patterned bars) at 60 °C and acetone (black bars) at 50 °C, after 72 h of incubation.

9.3 Results and Discussion Table 9.3 Initial reaction rates and the fraction of ester present

under the form of monoester for the acylation of various polyhydroxylated natural compounds (30 mM) with vinyl butyrate catalyzed by immobilized CALB in [bmim]BF4 and [bmim]PF6 at 60 °C and in acetone at 50 °C after 72 h of reaction. Substrates

Naringin Esculin Helicin Salicin Silybin

Initial reaction rates (mmol/h/g of biocatalyst) (mol% of monoester produceda) [bmim]BF4

[bmim]PF6

Acetone

0.06 (86.8) 0.04 (98.1) 0.15 (68.3) 0.08 (23.6) 0.27 (100.0)

0.05 (63.0) 0.03 (80.6) 0.10 (64.6) 0.05 (41.9) 0.07 (100.0)

0.04 (45.5) 0.03 (78.1) 0.03 (52.8) 0.03 (28.4) 1.58 (100.0)

a Refers to percentage of the total ester formed. The molar ratio of the substrates was 10.

2, 19.5, 12, and 8.3 mM, respectively. For the enzymatic acylation of all polyhydroxylated compounds studied in the present work, higher reaction rates were observed in ionic liquid media where the solubility of the substrates was higher, which is in accordance with the results reported by Park and Kazlauskas [16] for the enzymatic acetylation of glucose. The monoacylated derivative (6″-O-butyrate) was detected as the major product by HPLC analysis for all enzymatic reactions, while a diacylated derivative was also detected and confirmed by MS analysis [17]. As it can be seen in Table 9.3, higher regioselectivity of the process was observed in [bmim]BF4 than in [bmim]PF6, while the selectivity was even lower in acetone. The lower regioselectivity of the enzymatic acylation of polyhydroxylated compounds in [bmim]PF6 as well as in the organic solvent could be related to the lower solubility of unmodified phenolic substrates in these media, compared to that of their monoacylated derivatives [5, 16, 17]. On the other hand, the enhanced solubility of phenolic substrates in [bmim]BF4 could explain the increased regioselectivity observed in this ionic liquid. The enhanced solubility of all natural polyhydroxylated compounds tested in ionic liquids as compared to traditional organic solvents used for the enzymatic modification of these compounds can be highly useful for the production of great amounts of their acylated derivatives in a single-step biocatalytic process. It is noteworthy that, when the concentration of the above phenolic substrates was near their solubility limit, the amount of their monoacylated derivatives formed in [bmim]BF4 reached values up to 30.0 g/l (in the case of naringin), which were considerably higher than those reported for organic media [10, 30].

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Figure 9.3 Effect of the molar ratio of vinyl butyrate to silybin (30 mM) on the conversion yield for its enzymatic acylation catalyzed by immobilized CALB in the ionic liquids [bmim]BF4 and [bmim]PF6 at 60 °C after 72 h of incubation.

9.3.2.2 Effect of Substrate Concentration The effect of the molar ratio of vinyl butyrate to silybin on the conversion yield in the ionic liquids [bmim]BF4 and [bmim]PF6 is depicted in Figure 9.3. As can be seen in both ionic liquids used, the increase in the molar ratio of the substrates from 3 to 15 had a positive effect on the conversion yield. The same positive effect was also observed in the case of naringin (data not shown) and can be attributed to a thermodynamic shift of the equilibrium towards the synthesis of products due to acyl donor excess [6, 29]. In addition to the molar ratio increase, the increase in the antioxidant concentration was observed to have a positive effect on the conversion yield using both silybin and naringin (data not shown). 9.3.2.3 Effect of Acyl Donor Nature: Synthesis of Hybrid Antioxidants In order to investigate the effect of the nature of the acyl donor on the enzymatic acylation of naringin or silybin in ionic liquids several free fatty acids as well as their corresponding vinyl or methyl esters were used as acyl donors. The results show that, both in the case of naringin and silybin, the acyl donor chain length greatly affects the conversion yield of the enzymatic process. Higher conversion yields were achieved in both ionic liquid media using short-chain acyl donors (up to four carbon atoms), while the use of long-chain acyl donors resulted in substantially lower conversion yields (Table 9.4). However, in the case of acetone (results shown in Table 9.4) the enzyme shows a preference towards intermediate or high chain length fatty acids, which is in accordance with other reports for the enzymatic acylation of polyhydroxylated compounds catalyzed by immobilized CALB in conventional organic solvents

9.4 Conclusions Table 9.4 Effect of the acyl donor chain length on the

enzymatic acylation of silybin (30 mM) catalyzed by immobilized CALB in the ionic liquids [bmim]BF4 and [bmim]PF6 at 60 °C and in acetone at 50 °C after 72 h of incubation. Acyl donors

Caprylic acid Lauric acid Palmitic acid Oleic acid Vinyl butyrate Vinyl laurate Methyl oleate

Conversion (%) [bmim]BF4

[bmim]PF6

Acetone

8.9 5.8 1.1 1.6 26.5 3.9 2.3

9.8 6.4 1.5 13.6 23.9 3.8 23.7

12.4 21.5 28.4 14.2 53.6 75.4 24.6

The substrates were used in equimolar amounts.

[10, 23]. The different enzyme behavior observed in the case of ionic liquids can be attributed to the lower solubility of long chain acyl donors in these media, compared to the less polar organic solvents used for the enzymatic modification of natural polyhydroxylated compounds. Due to the low solubility of long chain acyl substrates in ionic liquids, a two-phase system was formed, which is expected to decrease the availability of substrates to the enzyme and therefore the biocatalytic acylation of phenolic compounds [5]. In addition to free fatty acids and their esters as previously described, compounds with antioxidant potency were also used as acyl donors. In order to investigate the synthesis of hybrid antioxidants, the enzymatic acylation of esculin catalyzed by immobilized CALB in the ionic liquids [bmim]BF4 and [bmim]PF6 was investigated using several phenolic acids as well as their corresponding vinyl esters. As can be seen in Figure 9.4, the enzymatic synthesis of hybrid antioxidants of esculin is feasible in ionic liquids, resulting in good conversion yields. The conversion yields obtained were higher in ionic liquid [bmim]PF6 as compared to [bmim]BF4 for all phenolic acids tested, while the use of phenolic acid esters in transesterification reactions led to higher conversion yields in both ionic liquid media used.

9.4 Conclusions

The biocatalytic acylation of polyhydroxylated natural compounds, including flavonoid and phenolic glucosides as well as flavonolignans, is feasible in both

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Figure 9.4 Conversion yields for the enzymatic acylation of esculin (25 mM) with different phenolic acids (50 mM) catalyzed by immobilized CALB in (a) [bmim]BF4 (black bars) and (b) [bmim]PF6 (patterned bars) after 72 h of incubation at 60 °C.

organic solvents and ionic liquid media. A variety of acyl donors such as dicarboxylic acids, free fatty acids, or their vinyl esters, as well as phenolic acids, which are compounds with antioxidant potency, can be used for the regioselective acylation of polyhydroxylated natural compounds and the preparation of lipophilic derivatives with biological activity. The effectiveness and the regioselectivity of the biocatalytic modification depend strongly on various reaction parameters, including the nature of the reaction medium, the nature of the acyl donor, the substrate concentrations, and the solubility of the substrates. The use of ionic liquids, such as [bmim]BF4, in which the solubility of natural polyhydroxylated compounds is greatly enhanced, can be advantageous for the production of larger amounts of lipophilic derivatives in a single-step biocatalytic process.

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4 Rice-Evans, C., Miller, N.J. and Paganga, G. (1996) Free Radical Biology & Medicine, 20, 933–956. 5 Katsoura, M.H., Polydera, A.C., Tsironis, L., Tselepis, A.D. and Stamatis, H. (2006) Journal of Biotechnology, 123, 491–503. 6 Mellou, F., Lazari, D., Skaltsa, H., Tselepis, A.D., Kolisis, F.N. and Stamatis,

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20 Wilkes, J.S. (2004) Journal of Molecular Catalysis A – Chemical, 214, 11–17. 21 Jarstoff, B., Störmann, R., Ranke, J., Mölter, K., Stock, F., Oberheitmann, B., Hoffmann, J., Nüchter, M., Ondruschka, B. and Filser, J. (2003) Green Chemistry, 5, 136–142. 22 Ranke, J., Mölter, K., Stock, F., BottinWeber, U., Poczobutt, J., Hoffmann, J., Ondruschka, B., Filser, J. and Jastorff, B. (2003) Ecotoxicology and Environmental Safety, 58, 396–404. 23 Kontogianni, A., Skouridou, V., Sereti, V., Stamatis, H. and Kolisis, F.N. (2003) Journal of Molecular Catalysis B: Enzymatic, 21, 59–62. 24 McCabe, R.W. and Taylor, A. (2004) Enzyme and Microbial Technology, 35, 393–398. 25 Uppenberg, J., Ohmer, N., Norin, M., Hult, K., Kleywegt, G.J., Patkar, S., Waagen, V., Anthomen, T. and Jones, T.A. (1995) Biochemistry, 34 (51), 16838–16851. 26 Teng, R.W., Bui, T.K.H., McManus, D., Armstrong, D., Mau, S.L. and Bacic, A. (2005) Biocatalysis and Biotransformation, 23, 109–116. 27 Nakajima, N., Ishihara, K., Itoh, T., Furuya, T. and Hamada, H. (1998) Journal of Bioscience and Bioengineering, 87 (1), 105–107. 28 Stamatis, H., Sereti, V. and Kolisis, F.N. (2001) Journal of Molecular Catalysis B: Enzymatic, 11, 323–328. 29 Kontogianni, A., Skouridou, V., Sereti, V., Stamatis, H. and Kolisis, F.N. (2001) European Journal of Lipid Science and Technology, 103, 655–660. 30 Otto, R.T., Scheib, H., Bornscheuer, U.T., Pleiss, J., Syldatk, C. and Schmid, R.D. (2000) Journal of Molecular Catalysis B: Enzymatic, 8, 201–211.

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10 Biocatalysis Applied to the Synthesis of Nucleoside Analogs Vicente Gotor

10.1 Introduction

Nowadays, many research groups employ biotransformations for preparing different kinds of organic compounds that are difficult to prepare by conventional chemical procedures. It is well recognized that biocatalysis is of special relevance in fine-chemical industry for manufacturing enantiopure compounds. Among the enzymes used for organic synthesis, lipases have demonstrated a great versatility in enzymatic hydrolysis, transesterification, or aminolysis reactions [1]. Other biocatalysts, such as oxidoreductases and some lyases, are also of interest. Of special relevance is the use of these biocatalysts for the preparation of pharmaceuticals [2]. Selective biotransformations of polyhydroxylated compounds, such as carbohydrates, have been used since some time ago for the activation or protection of some of their hydroxyl groups, allowing different chemical transformations to be carried out without the need for using tedious protection and deprotection steps. Monoesters of some monosaccharides and disaccharides have been obtained using lipases or proteases in polar organic solvents [3]. In addition, the utility of biotransformations in other natural products such as nucleosides [4] or steroids [5] has also been reviewed. Modified nucleosides have attracted much attention as antiviral and anticancer agents. Consequently, extensive changes have been made to both the heterocyclic base and the sugar moiety in order to reduce the toxicity and viral drug resistance associated with certain nucleoside analogs. Until recently, only nucleosides possessing the natural β-D configuration have been studied as chemotherapeutic agents due to their easy access. However, the discovery of lamivudine (an α-L deoxynucloside), the first compound with the unnatural β-L configuration approved by the Food and Drug Administration for use in combined therapy against human immunodeficiency virus type 1 (HIV-1) and hepatitis B virus (HBV), has sparked tremendous interest in the synthesis of β-L-nucleosides. As a result, several Lnucleosides are currently undergoing clinical trials as potential antiviral or antitumor agents [6]. Figures 10.1 and 10.2 show some nucleosides of clinical interest. Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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10 Biocatalysis Applied to the Synthesis of Nucleoside Analogs

Figure 10.1 Partial list of FDA-approved anti-HIV drugs.

Figure 10.2 Some L-nucleosides in clinical trials as anti-HBV.

In recent years biotransformations have also shown their potential when applied to nucleoside chemistry [7]. This chapter will give several examples that cover the different possibilities using biocatalysts, especially lipases, in order to synthesize new nucleoside analogs. The chapter will demonstrate some applications of enzymatic acylations and alkoxycarbonylations for the synthesis of new analogs. The utility of these biocatalytic reactions for selective transformations in nucleosides is noteworthy. In addition, some of these biocatalytic processes can be used not only for protection or activation of hydroxyl groups, but also for enzymatic resolution of racemic mixtures of nucleosides. Moreover, some possibilities with other biocatalysts that can modify bases, such as deaminases [8] or enzymes that catalyze the synthesis of new nucleoside analogs via transglycosylation [9] are also discussed.

10.2 Chemoenzymatic Modification of the Sugar

As modified oligonucleotides have become a major field of investigation for chemists, methods for the suitable protection/deprotection applied to the synthesis of nucleoside monomers have become equally important. Selective protection of a multifunctional compound is a challenging problem in organic synthesis. In nucleoside chemistry, selective manipulation of the hydroxyl groups on the carbohydrate moiety over amino groups of the bases is synthetically challenging and


requires a multistep protocol. In this sense, enzymes are a good alternative since biocatalyzed processes are environmentally acceptable working under mild conditions demonstrating high chemo- and regioselectivity and are usually compatible with organic solvents showing recycling possibilities. This first section will study different regioselective processes of several types of nucleosides depending on the lipase used. Application of biotransformations over these compounds has acquired great importance in order to prepare new derivatives with interesting pharmacological activities. Two lipases, namely, Candida antarctica type B (CALB) and Pseudomonas cepacea, free (PSL) or Pseudomonas cepacea, immobilized (PSLC), are selective towards one of the two hydroxyl groups of different 2′-deoxynucleosides. Thus, it is possible to prepare the acylated compounds in 5′-position with CALB [10], whereas PSL is selective towards the secondary hydroxyl group [11]. Vinyl or oxime esters can be used as acyl donors. The first example of the application of this process with different oxime esters is shown in Scheme 10.1. The solvent plays an important role in this process and dioxane is usually used in these reactions. It is remarkable that, in the case of ribonucleosides (X=OH), it is not possible to achieve high regioselectivities in the 2′ or 3′-position with PSL, but CALB again catalyzes the process towards the primary hydroxyl group in the 5′-position. In addition, a theoretic study to explain the abnormal regioselectivity of PSL to the less reactive hydroxyl group has recently been published [12]. Another application is that the introduction of functional groups of physiological interest, such as carbamates, can be carried out from the corresponding carbonates (Scheme 10.2). For the synthesis of the starting carbonates a similar strategy to the regioselective enzymatic acylation of 2′-deoxynucleosides is used: in this case oxime carbonates are employed as alkoxycarbonylating agents [13]. The synthesis of carbamates takes place using the corresponding vinyl carbonate derivatives, because these activated carbonates can react in the absence of catalyst with amines or ammonia to obtain the nucleoside–carbamate analogs in quantitative yields [14]. This enzymatic alkoxycarbonylation is of great utility in activating hydroxyl groups

Scheme 10.1 Regioselective acylation of 2′-deoxynucleosides.

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Scheme 10.2 Regioselective preparation of carbamate derivatives of 2′-deoxynucleosides.

Scheme 10.3 Chemoenzymatic preparation of amino-xylonucleoside derivatives.

or also protecting them when benzyloxime carbonate (R1=CH2Ph) is used as the alkoxycarbonylating agent. Moreover, this chemoenzymatic process yields new aminonucleosides that are very difficult to prepare by chemical procedures alone [15]. Scheme 10.3 shows a general chemoenzymatic procedure for obtaining 3′-amino-xylo-nucleosides. The key step is based on the 5′-directed intramolecular nucleophilic substitution at the 3′-activated position of the nucleoside. This methodology is applicable to ribonucleosides and 2′-deoxyribonucleosides and shows the utility of a combination of


biocatalysis and chemical catalysis for the preparation of products of high added value. Among the limited protecting groups available, the levulinyl group is frequently chosen to protect the 3′- and/or 5′-hydroxyl of the nucleosides for solution-phase oligonucleotide synthesis. The strategy for the synthesis of 3′-O-levulinyl nucleosides (2′-deoxy or 2′-O-protected) first involves protection of the 5′-hydroxy group as a 5′-O-dimethoxytrityl (5′-O-DMTr) derivative. Subsequent treatment with levulinic acid or levulinic anhydride and dicyclohexylcarbodiimide (DCC) followed by removal of the 5′-O-DMTr group under acidic media affords the 3′-protected nucleosides [16]. On the other hand, direct reaction of the corresponding unprotected nucleosides with levulinic acid, followed by separation of the 3′,5′-diacylated products by column chromatography and treatment of the residue with DMTrCl to remove the 3′-acylated compound and purification again by flash chromatography, furnished the 5′-O-levulinyl derivatives in low yields. A short and convenient synthesis of 3′- and 5′-O-levulinyl-2′-deoxynucleosides [17] has been developed in Scheme 10.4 from the corresponding 3′,5′-di-O-levulinyl derivatives by regioselective enzymatic hydrolysis, avoiding several tedious chemical protection/deprotection steps. Candida antarctica lipase A (CALA) has been identified as an ideal biocatalyst for the resolution of sterically hindered compounds [18] and in some cases has shown an opposite enantioselectivity to CALB [19]. In addition, the regioselective acylation of 2′-deoxynucleosides showed opposite regioselectivity using either CALA or CALB. Moreover, these compounds can also be prepared through enzymatic acylation from the natural nucleosides using the corresponding oxime ester derivative as the acyl donor (Scheme 10.5). This process takes place with high regioselectivity and excellent yields [20]. In summary, the enzymatic procedure is an efficient and regioselective method for the synthesis of 3′- or 5′-O-levulinyl-protected nucleosides. As a result, these key building blocks needed for solution-phase synthesis of oligonucleotides are easily accessible in high yield and purity. In some cases vinyl esters have been successfully used for these regioselective processes (Scheme 10.6). Thus, CALB catalyzes the selective monobenzoylation at the 5′-hydroxyl group of 2′-deoxynucleosides using vinyl benzoate as the acyl

Scheme 10.4 Enzymatic hydrolysis of di-O-levulinyl nucleosides.

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Scheme 10.5 Preparation of monolevulinyl 2′-deoxynucleosides through enzymatic acylation.

Scheme 10.6 Synthesis of 5′-benzoyl-2′-deoxynucleosides.

transfer reagent in quantitative yields [21]. In this case the application of acetonoxime benzoate as an acylating agent for other nucleosides furnished moderate yields with slow reaction rates. The easy scalability of the process and the fact that both the acylating agent and enzyme can be recovered and reused after each reaction makes the new process atom efficient [22] and very attractive for industrial applications. The overall approach for the synthesis of 3′-O-DMTr derivatives has been carried using these benzoyl derivatives as starting material. Introduction of the DMTr group at the 3′-hydroxyl position is accomplished by treatment of 5′-O-benzoyl-2′deoxynucleosides with DMTrCl in pyridine at 70 °C. The choice of this high reaction temperature furnished an improved reaction rate and better overall yield. Next, treatment with sodium methoxide in MeOH at 0 °C removed the 5′-Obenzoyl group [23]. Under these conditions, 3′-O-DMTr-thymidine is isolated in a 90% yield after chromatography (Scheme 10.7). However, under similar reaction conditions the N-benzoyl-cytidine derivative furnished a mixture of N-benzoyl-3′-O-DMTr-deoxycytidine and the corresponding N-unprotected nucleoside. However, this limitation was avoided using the corresponding levulinic derivative because the protecting group was easily removed using hydrazine (Scheme 10.8). It has been described that, in some cases, the acylation of one hydroxyl group of the sugar moiety in a nucleoside derivative can increase its biological activity


Scheme 10.7 Syntheses of 3′-O-DMTr 2′-deoxynucleosides.

Scheme 10.8 Syntheses of 3′-O-DMTr derivatives from 5′-O-levulinyl derivatives.

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Scheme 10.9 Synthesis of O-crotonyl 2′-deoxynucleosides.

Scheme 10.10 Chemical preparation of 3′,5′-diamino-3′,5′-trideoxythymidine.

compared with the unmodified analog [24]. Regioselective syntheses of several Ocrotonyl 2′-deoxynucleoside derivatives have been efficiently achieved using a biocatalytic methodology. While CALB affords the 5′-O-acylated compounds, PSLC provides the 3′-O-crotonylated analogs [25]. Since classical chemical approaches did not work appropriately due to side isomerization reactions, a mixture of both lipases was used for achieving a useful synthetic route toward diacylated nucleosides (Scheme 10.9). Aminonucleosides can offer interesting physiological properties. Some routes for the synthesis of new analogs of this kind of compounds have been described [26].A modified route for the preparation of 3′,5′-diamino-3″,5′-trideoxythymidine is outlined in Scheme 10.10. The reaction, which takes place in four steps with an overall yield of 63%, is an improvement on the earlier described process [27]. In addition, regioselective enzymatic acylation of pyrimidine 3′,5′-diaminonucleoside derivatives is possible depending on the biocatalyst employed in the process [28]. N-5′-Acylated products were obtained using CALB as a biocatalyst, whereas PSLC was selective towards the N-3′ position. Molecular modeling studies

10.3 Resolution and Anomeric Separation

Scheme 10.11 Enzymatic alkoxycarbonylation of 3′,5′-diaminonucleosides.

were developed in order to explain the behavior of CALB in these enzymatic processes [29]. On the other hand, alkoxycarbonylation of amines yields carbamates that are valuable intermediates in medicinal chemistry, as this functionality is part of various therapeutic agents since it allows, for example, a better permeability across the cellular membranes. Moreover, these processes also allow the protection of the amino group in mild conditions. The first regioselective enzymatic alkoxycarbonylation of primary amino groups was achieved with pyrimidine 3′,5′-diaminonucleosides using CALB and non-activated homocarbonates allowing the synthesis of several N-5′-carbamates with moderate to high yields (Scheme 10.11) [30].

10.3 Resolution and Anomeric Separation

Parallel kinetic resolution (PKR), a concept that has been introduced for reactions where starting from a racemic mixture can allow the preparation of two different compounds at the same reaction rate [31], has been applied for the separation of a mixture of β-D/L-deoxynucleosides. A practical synthesis of β-L-3′- and β-L-5′-Olevulinyl-2′-deoxynucleosides has been described for the first time [32] through enzymatic acylation and/or hydrolysis processes. It is remarkable that the different behavior exhibited by PSL in the acylation of D- and L-nucleosides allows the parallel kinetic resolution of D/L nucleoside racemic mixtures. Scheme 10.12 shows a PKR of a 1 : 1 mixture of D and L nucleosides via an acylation reaction for furnishing easily separable compounds. This methodology would have tremendous potential for both research and industrial applications in the nucleic acid field. In addition, a similar strategy allows the separation of α- and β-anomers of a mixture. Thus, an efficient and high yield protocol for the synthesis and separation of 3′- and/or 5′-protected α-2′-deoxynucleosides has been developed through regioselective acylation/deacylation processes catalyzed by enzymes [33]. PSLC was found to be highly chemo- and regioselective toward the 3′-position of the β-2′-deoxynucleoside derivatives, whereas the same lipase displayed opposite regioselectivity towards the 5′-position for the corresponding α-anomer (Scheme 10.13). The different Rf of both acylated anomers allows an easy separation.

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Scheme 10.12 Enzymatic separation of D/L-thymidine racemic mixture.

Scheme 10.13 Enzymatic separation of a α/β-anomeric mixture.

Scheme 10.14 Preparation of β-thymidine from a mixture of α/β-anomers.

A practical example of separation of a mixture of α/β-anomers of the mother liquor collected from the commercial-scale synthesis of β-thymidine is shown in Scheme 10.14. PSLC is used as a biocatalyst because this lipase has shown the desired opposite regioselectivity toward the hydrolysis of α- and β-nucleosides. The hydrolysis reaction takes place with excellent selectivity toward the 5′-p-

10.4 Biotransformations that Modify the Base

chlorobenzoate group of the α anomer furnishing 3′-O-(p-chlorobenzoyl)-α-Dthymidine as an exclusive product without a detectable amount of completely hydrolyzed α-thymidine. Despite the long reaction time, the β anomer is recovered unmodified. The pure α-nucleoside was isolated in 67% yield after chromatography. Next, the hydrolysis reaction with CALB was tried for two reasons: firstly, to show that CALB could be recycled several times to reduce the overall cost of the process and, secondly, CALB is commercially available at a reasonable price [33]. The hydrolysis of a mixture of the products with CALB under similar conditions was found to be faster than the PSLC-mediated reaction and afforded the same products. A complete conversion of the starting material to the products with CALB in 104 h instead of the 164 h required for the reaction with PSLC was observed. The faster reaction rate with CALB was expected to reduce the overall cycle time in the plant for the hydrolysis process and assist in meeting thelower cost objectives. After chromatographic separation, the conventional basecatalyzed hydrolysis of the products furnished β-thymidine and α-thymidine in excellent yields.

10.4 Biotransformations that Modify the Base

Due to the importance of modified nucleosides that are also in the base, the development of new biocatalytic processes applied to the synthesis of derivatives modified in the base is of great interest. Adenosine deaminase (ADA) and adenylate deaminase (AMPDA) are biocatalysts that catalyze the hydrolytic deamination of purine nucleosides and nucleotides. Some applications of these deaminases for the preparation and transformation of compounds structurally related to nucleosides with potential antitumor and antiviral activities have been described in the last few years [8]. ADA and AMPDA are enzymes involved in the purine cycle and they catalyze the hydrolytic deamination of purine nucleosides and nucleotides. In the last few years these biocatalysts have shown a great utility in biocatalysis, because they display broad substrate specificity and their use can also be extended to carbocyclonucleosides and acyclonucleosides [34]. The general process that takes place using these enzymes is outlined in Scheme 10.15.

Scheme 10.15 Hydrolytic deamination of ADA and AMPDA of adenosine and adenylic acid.

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ADA is able to transform other 6-substituted purine ribosides containing halogen, methoxy or hydroxylamino groups in inosine, although at lower rate [35]. In addition, the influence of the 5′-hydroxyl group of the sugar moiety is remarkable. Thus, it was demonstrated some time ago that 5′-O-protected or 5′-deoxy-5′substituted nucleosides are not adequate substrates for this enzyme [36]. However, protected nucleosides on the 2′- and 3′-position, such as the corresponding 2′,3′-isopropylidene adenosines [37], are adequate substrates when 5′-hydroxy or 5′-amino groups are present in the nucleoside, although in the latter case the process takes place with more difficulty (Scheme 10.16). Deaminases can be very practical for the transformation of nucleosides in other derivatives in combination with chemical catalysis. An interesting example is the synthesis of 2′,3′-dideoxyinosine starting from 2′-deoxyadenosine. The process takes place through a combination between two biocatalytic reactions followed by conventional chemical reactions in a multistep process. First, two enzymatic processes take place (Scheme 10.17). Again, CALB was the most effective catalyst in achieving the regioselective acylation in the 5′-position in comparison with other lipases [38]. Deoxygenation of the unprotected 3′-hydroxyl group was achieved

Scheme 10.16 Deamination of 2′,3′-isopropylidene derivatives.

Scheme 10.17 Synthesis of 2′,3′-dideoxyinosine in a multistep process.

10.5 Transglycosylation for the Synthesis of Nucleosides

Scheme 10.18 Enzymatic resolution of (±)-carbovir.

chemically by the formation of the corresponding phenylthiocarbonate and further reaction with tributyltin hydride. This chemical process is frequently used for the deoxygenation of different types of nucleosides [39]. In addition, ADA has also been used in the resolution of some nucleoside analogs derived from 6-aminopurines [40]. In this context, an interesting example is shown in Scheme 10.18. This strategy has been successfully developed for carrying out the resolution of (±)-carbovir. Transformation of the unmodified (+)-amino compound under different reaction conditions allows the preparation of (+)-carbovir, which is less active as an HSV-1 inhibitor than the levo enantiomer (Scheme 10.18) [41].

10.5 Transglycosylation for the Synthesis of Nucleosides

In this section some examples are discussed in order to prepare nucleoside analogs by base interchange through a transglycosylation reaction. In these processes two different types of intramolecular enzymes are usually employed: nucleoside phosphorylases and N-2′-deoxyribosyl transferases. Nucleoside phosphorylases catalyze the reversible phosphorolysis in nucleosides and the transferase reaction involving purine or pyrimidine bases [42]. Scheme 10.19 shows the general synthetic strategy of these processes. Although, nucleoside phosphorylases show a broad substrate specificity, N-2′-deoxyribosyl transferases specifically catalyze the exchange of the base from a 2′-deoxyribosylnucleoside with another purine or pyrimidine [43]. Two different mechanisms involved in the synthesis of 2′-deoxyribonucleosides by means of base interchange have been proposed. One of them involves a covalent catalysis analog to the one known for

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Scheme 10.19 Synthesis of nucleoside analogs by using nucleoside phosphorylases.

Scheme 10.20 Preparation of nucleoside analogs using E. coli strains.

glycosidases, where either a glutaryl or an aspartyl residue is involved in the catalysis [44]. The other possibility involves two different enzymes: thymidine nucleoside phosphorylase and purine nucleoside phosphorylase [45]. The preparation of different nucleoside analogs by interchange purine– pyrimidine or pyrimidine–purine bases has been well documented [46]. Scheme 10.20 depicts the versatility of this reaction using whole cells as biocatalysts. In

10.6 Summary

Scheme 10.21 Preparation of nucleoside analogs through a multistep process.

this case, the Escherichia coli strain presents great versatility for the preparation of different nucleoside analogs [47]. In some cases the combination of two biocatalysts in a multistep process allows, in an indirect manner, the preparation of some compounds or increases in their yields. Thus, Scheme 10.21 shows that the synthesis of a nucleoside analog containing a hypoxanthine base takes place in a two-step fashion using a combination of both nucleoside phosphorylase and adenine deaminase, obtaining the final analog in high yield. However, the direct preparation of this compound with hypoxanthine takes place with a very low yield due to the low solubility of this base [48].

10.6 Summary

The possibility of carrying out enzymatic processes under mild conditions and their high selectivity make biocatalytic reactions very attractive for performing some transformations that are difficult to achieve by chemical procedures alone. This chapter has shown the utility of three different types of enzymes for selective transformations on nucleosides applied to the preparation of new analogs. It is well established that lipases are the biocatalysts mostly employed by organic chemists, especially in the resolution of racemic mixtures. This chapter has reviewed some of the processes concerning the versatility of lipases for the regioselective protection or activation of hydroxyl groups in deoxynucleosides. In addition, in recent years some applications for the resolution of D/L mixtures and separation of anomers have emerged and these processes will probably have great applicability in industry, since lipases are able to carry out this separation in mild conditions. Moreover, in the last few years two types of enzymes, deaminases, which convert

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6-substituted purine nucleosides into the corresponding 6-oxo analogs and glycosyltransferases, which carry out the preparation of nucleoside analogs by base interchange through a transglycosylation reaction, have emerged as useful biocatalysts for the synthesis of new derivatives of physiological interest.

References 1 Gotor-Fernández, V. and Gotor, V. (2007) Use of lipases in organic synthesis, in Industrial Enzymes: Structure, Function and Application (eds J. Polaina and A.P. MacCabe), Springer, Dordrech, The Netherland, Chapter 18. 2 Gotor, V. (2002) Organic Process Research & Development, 6, 420–426. 3 Wong, C.-H. (1995) Pure and Applied Chemistry, 67, 1609–1616. 4 (a) Ferrero, M. and Gotor, V. (2000) Monatshefte für Chemie, 131, 585–618. (b) Ferrero, M. and Gotor, V. (2000) Chemical Reviews, 100, 4319–4348. 5 Ferrero, M. and Gotor, V. (2000) Biocatalytic synthesis of steroids, in Stereoselective Biocatalysis (ed. R.N. Patel), Marcel Dekker, New York, USA, Chapter 11. 6 (a) Gumina, G., Chong, Y., Choo, H., Song, G.-Y. and Chu, C.K. (2002) Current Topics in Medicinal Chemistry, 2, 1065–1086. (b) Sommadossi, J.-P. (2002) Recent Advances in Nucleosides (ed. C.K. Chu), Elsevier, Amsterdam, pp. 417–432. (c) Lee, K. and Chu, C.K. (2001) Antimicrobial Agents and Chemotherapy, 45, 138–144. (d) Standing, D.N., Bridges, E.G., Placidi, L., Faraj, A., Loi, A.G., Pierra, C., Dukhan, D., Gosselin, G., Imbach, J.-L., Hernández, B., Juodawlkis, A., Tennant, B., Korba, B., Cote, P., Cretton-Scott, E., Schinazi, R.F., Myers, M., Bryant, M.L. and Sommadossi, J.-P. (2001) Antiviral Chemistry & Chemotherapy, 12, 119–129. (e) Gumina, G., Song, G.-Y. and Chu, C.K. (2001) FEMS Microbiology Letters, 202, 9–15. 7 Lavandera, I., García, J., Fernández, S., Ferrero, M., Gotor, V. and Sanghvi, Y. (2005) Current protocols in nucleic acid chemistry, in Protection of Nucleosides for

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Oligonucleotide Synthesis, John Wiley & Sons, Ltd, 2.11.1–2.11.36. Santaniello, E., Ciuffeda, P. and Alessandrini, L. (2007) Deaminating enzymes of the purine cycle as biocatalysts for chemoenzymatic synthesis and transformations of antiviral agents structurally related to purine nucleosides, in Biocatalysis in the Pharmaceutical and Biotechnology Industries (ed. R. Patel), CRC Press, Chapter 17. Cordezo, L.A., Fernández-Lucas, J., García Burgos, C.A., Alcantara, A.R. and Sinisterra, J.V. (2007) Enzymatic synthesis of modified nucleosides in biocatalysis, in The Pharmaceutical and Biotechnology Industries (ed. R. Patel), CRC Press, Chapter 14. Morís, F. and Gotor, V. (1993) Journal of Organic Chemistry, 58, 653–660. Morís, F. and Gotor, V. (1992) Synthesis, 7, 626–628. Lavandera, I., Fernández, S., Magdalena, J., Ferrero, M., Grewal, H., Savile, C.K., Kazlauskas, R.J. and Gotor, V. (2006) ChemBioChem, 7, 693–698. (a) Morís, F. and Gotor, V. (1992) Journal of Organic Chemistry, 57, 2490–2492. (b) Morís, F. and Gotor, V. (1992) Tetrahedron, 48, 9869–9876. (a) García-Allés, L., Morís, F. and Gotor, V. (1993) Tetrahedron Letters, 34, 6337–6338. (b) García-Allés, L. and Gotor, V. (1995) Tetrahedron, 51, 307–316. García-Allés, L., Magdalena, J. and Gotor, V. (1996) Journal of Organic Chemistry, 61, 6980–6986. (a) Reese, C.B. and Song, Q. (1999) Nucleic Acids Research, 27, 963–971. (b) Reese, C.B. and Song, Q. (1999) Journal of the Chemical Society – Perkin Transactions, 1, 1477–1486. García, J., Fernández, S., Ferrero, M., Sanghvi, Y.S. and Gotor, V. (2002)

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Journal of Organic Chemistry, 67, 4513–4519. Domínguez de María, P., CarboniOerlemans, C., Tuin, B., Bargeman, G., Van de Meer, A. and Van Gemert, R. (2005) Journal of Molecular Catalysis B: Enzymatic, 37, 36–46. de Gonzalo, G., Brieva, R., Sánchez, V.M., Bayod, M. and Gotor, V. (2001) Journal of Organic Chemistry, 66, 8947–8953. García, J., Fernández, S., Ferrero, M., Sanghvi, Y.S. and Gotor, V. (2003) Tetrahedron: Asymmetry, 14, 3533–3540. García, J., Fernández, S., Ferrero, M., Sanghvi, Y.S. and Gotor, V. (2004) Tetrahedron Letters, 45, 1709–1712. Trost, B.M. (2002) Accounts of Chemical Research, 35, 695–705. Díaz-Rodríguez, A., Fernández, S., Sanghvi, Y.S., Ferrero, M. and Gotor, V. (2006) Organic Process Research & Development, 10, 581–587. (a) Hamamura, E.K., Prystasz, M., Verheyden, J.P.H., Moffat, J.G., Yamaguchi, K., Uchida, N., Sato, K., Nomura, A., Shiratori, O., Takese, S. and Katagiri, K. (1976) Journal of Medicinal Chemistry, 19, 654–662. (b) Hamamura, E.K., Prystasz, M., Verheyden, J.P.H., Moffat, J.G., Yamaguchi, K., Uchida, N., Sato, K., Nomura, A., Shiratori, O., Takese, S. and Katagiri, K. (1976) Journal of Medicinal Chemistry, 19, 667–674. Díaz-Rodríguez, A., Lavandera, I., Fernández, S., Sanghvi, Y.S., Ferrero, M. and Gotor, V. (2005) Tetrahedron Letters, 46, 5835–5838. Herdewijn, P., Balzarini, J., Pauwels, R., Janssen, G., Van Aerschot, A. and De Clercq, E. (1989) Nucleosides Nucleotides, 8, 1231–1257. Lavandera, I., Fernández, S., Ferrero, M. and Gotor, V. (2003) Tetrahedron, 59, 5449–5456. Lavandera, I., Fernández, S., Ferrero, M. and Gotor, V. (2001) Journal of Organic Chemistry, 66, 4079–4082. Lavandera, I., Fernández, S., Magdalena, J., Ferrero, M., Kazlauskas, R.J. and Gotor, V. (2005) ChemBioChem, 6, 1381–1390.

30 Lavandera, I., Fernández, S., Ferrero, M. and Gotor, V. (2004) Journal of Organic Chemistry, 69, 1748–1751. 31 Dehli, J.R. and Gotor, V. (2002) Chemical Society Reviews, 31, 365–370. 32 García, J., Fernández, S., Ferrero, M., Sanghvi, Y.S. and Gotor, V. (2004) Organic Letters, 6, 3759–3762. 33 García, J., Fernández, S., Ferrero, M., Sanghvi, Y.S. and Gotor, V. (2006) Journal of Organic Chemistry, 71, 9765–9771. 34 Santaniello, E., Ciuffreda, P. and Alessandrini, L. (2005) Synthesis, 4, 509–526. 35 Magire, M.-H. and Sim, M.K. (1971) European Journal of Biochemistry, 23, 22–29. 36 Cory, J.G. and Suhadolnik, R.J. (1965) Biochemistry, 4, 1729–1732. 37 Ciuffreda, P., Loseto, A. and Santaniello, E. (2002) Tetrahedron, 58, 5767–5771. 38 Ciuffreda, P., Casati, S. and Santaniello, E. (1999) Bioorganic and Medicinal Chemistry Letters, 9, 1577–1582. 39 Robins, M.J., Wilson, J.S. and Hansske, F. (1983) Journal of the American Chemical Society, 105, 4059–4065. 40 Moon, H.R., Ford, H. and Marquez, V.E. (2000) Organic Letters, 2, 3793–3796. 41 Vince, R. and Brownell, J. (1990) Biochemical and Biophysical Research Communications, 168, 912–916. 42 Pal, S. and Nair, V. (1997) Biocatalysis and Biotransformation, 15, 147–158. 43 Short, S.A., Amnstrong, S.R., Ealick, S.E. and Porter, D.J. (1996) Journal of Biological Chemistry, 271, 4978–4987. 44 Danzin, C. and Cardinaud, R. (1976) European Journal of Biochemistry, 62, 365-372. 45 Walter, M.R., Cook, W.J., Cole, L.B., Short, S.A., Koszalka, G.W., Krenitsky, T.A. and Ealick, S.E. (1990) Journal of Biological Chemistry, 265, 14016–14022. 46 Hene, W.J. and Wong, C.H. (1989) Journal of Organic Chemistry, 54, 4692–4695. 47 Shirae, H., Kobayashi, K., Shiragami, H., Irie, Y., Yasuda, N. and Yokozeki, K. (1989) Applied and Environmental Microbiology, 55, 419–424. 48 Yokozeki, K. and Tsuji, T. (2000) Journal of Molecular Catalysis B: Enzymatic, 10, 207–213.

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11 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus Francisco J. Plou, Miguel Alcalde, Iraj Ghazi, Lucía Fernández-Arrojo and Antonio Ballesteros

11.1 Introduction

Glycosyltransferases (EC 2.4) catalyze the transfer of a glycosyl donor to an acceptor molecule thereby forming a new glycosidic bond with great regio- and/or stereoselectivity [1, 2]. Glycosyltransferases are classified into three main mechanistic groups depending on the nature of the donor molecule: Leloir-type glycosyltransferases, which require sugar nucleotides (for example UDP-glucosyltransferases), non-Leloir glycosyltransferases, which use sugar-1-phosphates (for example phosphorylases) and transglycosidases, which employ non-activated sugars such as sucrose, lactose, or starch [3]. Transglycosidases present the same mechanism as retaining glycosidases, resulting in the net retention of the stereochemical anomeric configuration [4]. According to the Henrissat classification [5], which is based on amino acid sequence comparisons and comprises more than 2500 enzymes, both transglycosidases and glycosidases (EC 3.2) are grouped in the ‘glycoside hydrolase family’ (GH family). Although the normal function of transglycosidases is the transfer of a glycosyl residue to another sugar acceptor (transfer reaction), they can also use water as the acceptor of the glycosyl–enzyme intermediate (hydrolysis reaction). Only an enzyme that possesses a significant transglycosylation to hydrolysis ratio should be considered a transglycosidase [6]. Fructosyltransferases are transglycosidases that transfer the fructosyl moiety of sucrose yielding fructose oligomers (fructooligosaccharides, FOS) and/or polymers (fructans, for example inulin or levan) [7]. Enzymes with transfructosylating activity are present in many higher plants (asparagus, Jerusalem artichoke, chicory, onion, and so on) and microorganisms, (especially fungi [8–10]) and belong to families 32 and 68 of the GH family in the Henrissat classification [5]. The assignation of a particular fructooligosaccharide-producing enzyme to the fructosyltransferases (EC 2.4.1.9) or β-fructofuranosidases (invertases, EC 3.2.1.26) groups still remains controversial and should be based on its transferase to hydrolysis ratio, especially at low substrate concentrations [11].


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11 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus

Fructooligosaccharides are fructose oligomers with a terminal glucose group in which two to four fructofuranosyl moieties are linked via β(2 → 1)-glycosidic bonds [7, 12]. Commercial fructooligosaccharides are mainly composed of 1-kestose, nystose, and 1F-fructofuranosylnystose (Scheme 11.1) [13]. They are produced on a multiton scale from sucrose using transfructosylating enzymes from Aspergillus and related fungi (Aspergillus niger, Aspergillus oryzae, Aureobasidium pullulans, and so on) [7–10, 14]. There is also interest in the development of novel fructooligosaccharides with improved properties. In this context, 6G-type fructooligosaccharides (also called neo-fructooligosaccharides) consist mainly of neo-kestose (neo-GF2) and neo-nystose (neo-GF3), in which a fructosyl unit is β(2 → 6) bound to the glucose moiety of sucrose or 1-kestose, respectively [15, 16]. 6F-type fructooligosaccharides (6F-fructooligosaccharides) occur naturally in various food products: they are linear or branched β-(2,6)-linked fructooligosaccharides (the first is 6-kestose) [17, 18]. Recently, the regioselective enzymatic synthesis of 6F-type fructooligosaccharides with the β-fructofuranosidase from Schwanniomyces occidentalis has been

Scheme 11.1 Transfructosylation and hydrolytic reactions catalyzed by fructosyltransferase from A. aculeatus.

11.2 Purification of Fructosyltransferase in Pectinex Ultra SP-L

reported [19]. Fructooligosaccharides are non-cariogenic, have a sweetness of about 40–60% compared with sucrose, and, more importantly, exhibit prebiotic properties. In the context of functional foods, a prebiotic is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or the activity of certain types of bacteria in the colon (basically genus Bifidobacterium and Lactobacillus) [20, 21]. Prebiotics produce positive effects on human health as the metabolism of these bacteria releases short-chain fatty acids (acetate, propionate, and butyrate) and L-lactate [22, 23] with different implications for health. Between them, they exert protective effects against colorectal cancer and bowel infectious diseases by inhibiting putrefactive and pathogen bacteria, improve the bioavailability of essential minerals, or enhance the glucid and lipid metabolism [24, 25]. Besides the fructooligosaccharides [26] and galactooligosaccharides (GOS) [27], which are commercialized in the USA and Europe, Japan produces other ‘emerging’ prebiotics (isomaltooligosaccharides, soybean oligosaccharides, lactosucrose, gentiooligosaccharides, and xylooligosaccharides, to cite some) the market of which will be soon extended [28]. Pectinex Ultra SP-L is a commercial enzyme preparation from Aspergillus aculeatus that is used in the food industry for reducing viscosity in fruit juice processing. It contains different pectinolytic and cellulolytic enzymes [29]. In addition, the existence of fructosyltransferase activity in Pectinex Ultra SP-L has been reported by several authors [30–32]. In recent years, we have investigated the purification, characterization, and application of the fructosyltransferase from A. aculeatus contained in this commercial preparation [33].

11.2 Purification of Fructosyltransferase in Pectinex Ultra SP-L

The fructosyltransferase present in Pectinex Ultra SP-L was purified using a combination of DEAE-Sepharose, Mono-Q HR 5/5 and Sephacryl S-100 columns [33]. The fructosyltransferase was found to be a minor protein, as it only represented 0.4% (w/w) of the total protein in the extract. The enzyme was purified 107-fold from the crude Pectinex and the recovered activity was about 37%. The purified protein was blotted from an SDS–polyacrylamide gel electrophoresis (PAGE) gel to a polyvinylidene fluoride PVDF membrane and analyzed using an automatic sequencer for N-terminal amino acids. The peptide sequence for the first 20 amino acids was determined and compared with the sequence of known proteins using BLASTP (http://expasy.org/tools/blast). The results of the alignment are shown in Table 11.1. This amino acid sequence showed a significant homology with the N-terminal of β-fructofuranosidase from A. niger ATCC (American Type Culture Collection) 20611 [34] and with an invertase from A. niger B60 [35]. Its relative electrophoretic mobility in denaturing and non-denaturing polyacrylamide gels indicated that it had a dimeric structure with an estimated molecular mass of 65 kDa per monomer (Figure 11.1). This was confirmed by

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11 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus Table 11.1 Alignment of N-terminal amino acid sequences from A. aculeatus and A. niger fructosyltransferases and invertases.

Enzyme Fructosyltransferase from Aspergillus aculeatus Fructosyltransferase from Aspergillus niger ATCC 20611 Invertase from Aspergillus niger B60

N-terminal 1

LDTTAPPXFXLSTLPXXXLF

20

3

LDTTAPPPTNLSTLPNNTLF

22

6

DYNVAPP–NLSTLPNGSLF

23

Figure 11.1 Native polyacrylamide gel (5%) electrophoresis of purified fructosyltransferase. (a) Lane 1, molecular weight markers (45 kDa ovalbumin, 67 kDa bovine serum albumin (BSA), and 134 kDa BSA dimer); lane 2, pure fructosyltransferase. (b) Pure fructosyltransferase (one lane from the same gel was cut and stained for fructosyltransferase activity).

gel-filtration chromatography on Sephacryl S-100 using standard proteins. Fructosyltransferase purified from Pectinex Ultra SP-L was found to be a glycoprotein. The glycosylation degree was deduced from the difference between the molecular mass of the monomer and that of the deglycosylated form obtained after treatment with peptide-N-glycosidase F (PNGase F) (52 kDa, assessed by SDS-PAGE). This suggests that the enzyme contains about 20% (w/w) carbohydrate. The isoelectric point of the enzyme was estimated to be in the range 3.8–4.2 on the basis of relative mobility on a 4% polyacrylamide gel under denaturing conditions. Fructosyltransferases and β-fructofuranosidases from various sources show substantial differences in their molecular properties (subunit molecular weight,

11.3 Properties of Fructosyltransferase from A. aculeatus Table 11.2 Molecular properties of fructosyltransferases and β-fructofuranosidases from different sources.

Source

Subunit MW (kDa)a

Native MW (kDa)b

pI

Reference

Arthrobacter sp. K1 Asparagus officinalis L. A. aculeatus Aspergillus foetidus Aspergillus nidulans, S-form A. niger ATCC 20611 A. niger B60 A. niger IMI303386 A. niger AS0023 A. niger from mouldy lemons A. oryzae ATCC 76080 Azotobacter chroococum ATCC 4412 Bacillus macerans Bifidobacterium infantis ATCC 15697 Bifidibacterium infantis JCM 7007 C. utilis Clostridium perfringens Lactobacillus reuteri CRL 1100 S. cerevisae Schizosaccharomyces pombe Schwanniomyces occidentalis

52 64 65 90 78 100 115 125 125 47 87 57 66 70 75 150 37 58 60 205 77 85

52 64 134 180 185 340 230 125 250–750 95 87 59 66 70 232 300 37 58 120 205 150 85

4.3 – 4 – ∼5 – – ∼5 – – – – – 4.3 – – – – – – – –

[37] [38] [33] [39] [40] [41] [35] [42] [43] [44] [45] [46] [47] [48] [49] [36] [50] [51] [52] [53] [54] [19]

a Approximate molecular weight obtained from SDS-PAGE. b Approximate molecular weight obtained from native-PAGE or gel filtration.

isoelectric point, degree of deglycosylation, and so on). Table 11.2 summarizes some characteristics of the purified fructosyltransferase from A. aculeatus compared with related enzymes. Many of them are dimeric or multimeric enzymes: the monomer only shows activity in a few cases, for example in S. occidentalis [19] or in Candida utilis [36].

11.3 Properties of Fructosyltransferase from A. aculeatus 11.3.1 Substrate Specificity

The substrate specificity of the fructosyltransferase from A. aculeatus was estimated using a variety of di-, tri-, and tetrasaccharides at 100 g/l. The assay was

157

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based on the detection of reducing sugars by the dinitrosalicylic acid method [55] and was adapted to 96-well microplates [56]. As both the transglycosylation and hydrolytic reactions liberate reducing sugars (Scheme 11.1), the measured activity was a combination of both processes. The pure enzyme displayed the preference sucrose > raffinose > 1-kestose > nystose (in a ratio 100 : 24 : 10 : 6). Other carbohydrates such as turanose, cellobiose, melibiose, leucrose, methyl-α-D-glucopyranoside, and stachyose were also tested but their activity was negligible. Thus, the enzyme only recognized sugars containing a sucrose moiety in their chemical structure. The trisaccharide raffinose is equivalent to a sucrose molecule substituted at the C-6 hydroxyl group, whereas 1-kestose and nystose can be considered as different sucrose derivatives, fructosylated at the C-1′-position. The specific activity diminished with longer oligosaccharides, being negligible with the sucrose-containing tetrasaccharide stachyose, which suggests limitations for the binding of large molecules in the active site of this fructosyltransferase. 11.3.2 Effect of pH and Temperature

The optimum pH measured with sucrose as substrate ranged from 5.0 to 7.0, which is similar to that reported with other fructosyltransferases [7, 43, 57]. No significant activity was found at pH values below 3.5 and above 9.5. Regarding the pH stability, it was observed that the enzyme was more stable in the presence of carbohydrates, which mimics the operational conditions of fructooligosaccharide synthesis. Thus, the enzyme retained more than 90% of its initial activity after 24 h incubation at moderately acid and neutral pH values (from 4.5 to 7.5), whereas in the absence of the carbohydrate it retained less than 50% of the initial activity in the same pH range [33]. The optimum temperature was in the range 50–70 °C, which is slightly higher than that reported for related enzymes [58]. The enzyme was significantly stable up to 60 °C (t1/2 > 24 h), but temperatures greater than 65 °C inactivated the enzyme. 11.3.3 Influence of Chemicals

The fructosyltransferase from A. aculeatus displayed activity without the addition of any metal ions. However, some effects were observed in its susceptibility to mono- and divalent cations [33]. For example, Mn2+, K+, and Co2+ caused a 1.4–1.9fold increase in the activity, whereas low concentrations of Hg2+ and Zn2+ produced 35–60% inhibition. It was also found that the enzyme was slightly activated by several non-ionic and anionic surfactants such as sodium dodecylsulphate (1.5-fold at 10 mM), sodium deoxycholate (1.4-fold at 1 mM), and Triton X-100 (1.4-fold at 5% w/v). Moreover, it was resistant to low concentrations (1–10 mM) of reducing agents such as dithiothreitol and β-mercaptoethanol.

11.3 Properties of Fructosyltransferase from A. aculeatus

11.3.4 Kinetic Behavior

Both fructooligosaccharide synthesis and sucrose hydrolysis are catalyzed by most of the fructosyltransferases and β-fructofuranosidases (invertases) in the presence of sucrose. The transferase : hydrolase ratio, which determines the maximum yield of fructooligosaccharide, depends basically on two parameters: the concentration of sucrose and the intrinsic enzyme properties, that is its ability to bind the nucleophile (to which a fructose is transferred) and to exclude H2O from the acceptor binding site [11]. In order to investigate the behavior of fructosyltransferase from A. aculeatus in more detail, the kinetic constants (Km and kcat) for both transfer and hydrolysis were determined. The reaction rates (expressed in U/mg protein) versus sucrose concentration up to 1.75 M were measured (Figure 11.2). The reactions were analyzed by high-performance liquid chromatography (HPLC) to discriminate between hydrolytic and transferase activities. The hydrolytic and transfructosylating initial rates were calculated from the amount of free and transferred fructose, respectively. The latter was calculated by subtracting the amount of free fructose from the amount of glucose. As shown in Figure 11.2, the transglycosylation and hydrolysis profiles did not fit well with Michaelis–Menten kinetics. However, the adjustment improved substantially when applying the Hill transformation of the Michaelis– −1 Menten equation ⎡V = Vmax[S ]h (K mh + [S ]h ) ⎤ , where h is the Hill coefficient. This ⎣ ⎦ has been also observed with other fructosyl-transferring enzymes [59]. The Km and kcat values for the transferase and hydrolytic activities are given in Figure 11.2. With regard to the hydrolytic reaction, the Km value (27 mM) is in the range of those reported for β-fructofuranosidases from Saccharomyces cerevisae (26 mM) [60], Pichia anomala (16 mM) [61], or Arxula adeninivorans (41 mM) [62] and higher than those observed for invertases from S. occidentalis (4.9 mM) [19] or C. utilis (1–2 mM) [63]. However, the kcat for hydrolysis (775 s−1) was lower than those previously described for well-known invertases (for example 9430 s−1 for S. cerevisae invertase [60]). Although the Km for the transfer reaction (535 mM) was higher than those reported for enzymes exhibiting notable transfructosylating activity (for example 290 mM for A. niger fructosyltransferase [41]), the main feature of A. aculeatus fructosyltransferase was its significant kcat value for the transfer reaction (16 200 s−1). This implies that, at high sucrose concentrations (above 1 M, 342 g/l), the rate of transfer is nearly 20-fold that of hydrolysis, which explains the low concentration of free fructose that was observed with this enzyme in the HPLC analyses. 11.3.5 Fructooligosaccharide Production

Maximal fructooligosaccharide production for any particular enzyme depends basically on the relative rates of the transfructosylation and hydrolysis reactions

159

160


Figure 11.2 Kinetic plots (initial rate versus sucrose concentration) of fructosyltransferase from A aculeatus. Transfer activity (•) and hydrolytic activity (o). Reactions were carried out in 0.2 M sodium acetate buffer (pH 5.5) at 60 °C. Kinetic parameters were calculated estimating a molecular mass of 135 kDa for the active enzyme. Adapted from Ref. [33].

[42]. The pure fructosyltransferase from A. aculeatus was assayed for the production of fructooligosaccharides. A concentrated sucrose solution (600 g/l) containing pure enzyme (5 U/ml) was incubated at 60 °C and the transglycosylation products were investigated in detail by HPLC. A series of fructooligosaccharides (1-kestose, nystose, and 1F-fructosylnystose) was formed [56]. The analysis presented a maximum fructooligosaccharide production of 60.7% (w/w), referred to total carbohydrates in the reaction mixture. The rest of the carbohydrates in the mixture were fructose (marginal), glucose, and the remaining sucrose. These values compared well with those reported using the crude Pectinex Ultra SP-L under similar experimental conditions [32, 56] and with fructosyltransferases from other Aspergillus strains [8, 10, 64, 65].

11.4 Immobilization of Fructosyltransferase from A. aculeatus

11.4 Immobilization of Fructosyltransferase from A. aculeatus 11.4.1 Sepabeads EC-EP as Immobilization Carriers

For industrial application of carbohydrate-active enzymes, an effective immobilization method is required for facilitating the continuous processing and reuse of the biocatalyst [66, 67]. In this context, the use of available carriers for covalent immobilization is of great interest for minimizing enzyme leakage from the support, considering that these reactions take place in aqueous media. Sepabeads EC-EP are polymethacrylate-based carriers with a high density of reactive epoxide groups for protein covalent coupling. Compared with other acrylic epoxy-activated polymers, Sepabeads EC-EP possess a high mechanical and osmotic stability, low compressibility, and do not swell in water. Furthermore, the raw materials applied for the production of these supports are included in the European Union list of resins allowed for the processing of foodstuffs. Two epoxy-activated related supports, Sepabeads EC-EP3 and EC-EP5 (Figure 11.3), were assayed for the immobilization of A. aculeatus fructosyltransferase. By combination of nitrogen isotherms and mercury porosimetry analyses, the textural properties of both carriers were determined (Table 11.3). As shown, both samples

Figure 11.3 Scanning electron micrographs of Sepabeads ECEP05: (a) 60×, (b) 500×, and (c) 8000×.

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11 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus Table 11.3 Main properties of the Sepabeads EC-EP carriers.

Properties

EC-EP3

EC-EP5

Content of epoxide groups (µmol/g)a Average particle size (µm)b SBET (m2/g)c Density of epoxide groups (µmol/m2) Pore volume (cm3/g)d Average pore size (nm) Water content (%)e

106 77 43 2.5 1.19 130 60

23 139 9 2.4 1.67 800 65

a Provided by the supplier. b Determined by mercury porosimetry, considering a symmetric distribution of particle size. c Specific area measured by N2 adsorption. d By combination of N2 isotherms and mercury porosimetry. e Determined by Karl–Fisher titration.

were essentially macroporous, although the total pore volume of Sepabeads EC-EP5 (1.67 cm3/g) was significantly higher than that of Sepabeads EC-EP3 (1.19 cm3/g). The average pore size of both supports was also different: the maximum in the pore size distribution curve was 800 nm for Sepabeads EC-EP5 and 130 nm for Sepabeads EC-EP3. 11.4.2 Effect of pH and Ionic Strength on Immobilization

The coupling of enzymes to epoxy-activated carriers is commonly carried out at high ionic strength, because a salt-induced association between the macromolecule and the support surface increases the effective concentration of nucleophilic groups on the protein close to the epoxide reactive sites, thus favoring the immobilization process [68, 69]. However, the salt concentration needed for immobilizing an enzyme is highly dependent on the nature of the biocatalyst [69]. For the above reasons, the effect of pH and ionic strength on the immobilization of the fructosyltransferase from A. aculeatus on Sepabeads EC-EP was studied. Although this enzyme constitutes only a minor percentage (0.4% w/w) of the total proteins present in Pectinex Ultra SP-L, the commercial preparation was used directly (without enzyme purification) in order to develop a simple process that could be easily scaleable in the industry. It was considered that, although the other proteins present in Pectinex Ultra SP-L could also be immobilized in Sepabeads EC-EP, they would not interfere in fructooligosaccharide synthesis experiments. In order to bind the enzyme to the support using different functional groups, the immobilization was performed at two pH values (5.5 and 9.0), adjusting the pH of commercial Pectinex Ultra SP-L with potassium phosphate or sodium carbonate, respectively. At pH 5.5, the reactive groups of the protein are the carboxylic

11.4 Immobilization of Fructosyltransferase from A. aculeatus

heads of the aspartic and glutamic side chains, as well as the C-terminal αcarboxylic group [70]. At pH 9.0, the amino and thiol groups in the protein are able to bind to the support. The buffer concentration was varied over the range 0.2–1.0 M. A total of 30 mg of protein were mixed with 1 g of support. Figure 11.4 shows the protein recovery at 0.3 and 0.5 M buffer concentration after 24 h incubation. Surprisingly, the amount of protein bound to the support was higher at lower ionic strength. Maximums of 34 and 58% of the ‘offered’ protein were bound to Sepabeads EC-EP3 and EC-EP5, respectively. In general, Sepabeads EC-EP5 retained more protein than EC-EP3 under the same experimental conditions, probably as a consequence of its higher porosity. The effect of enzyme immobilization on the porosity of the supports was also analyzed: a slight decrease (approximately 4%) in the pore volume of Sepabeads EC-EP3 was produced as a consequence of enzyme immobilization. The total bound protein was larger at pH 9.0 than at pH 5.5. It is worth noting that a low buffer concentration also resulted in higher activity with both supports [56]. The highest activity of the biocatalyst (15.2 U/g) was achieved with Sepabeads EC-EP5, using 0.3 M sodium carbonate (pH 9.0). With Sepabeads EC-EP3, a slightly acidic pH (5.5) and low buffer concentration (0.3 M) yielded the highest biocatalyst activity (4.2 U/g). The incubation time was also analyzed: a further increase from 24 to 72 h did not result in a significant improvement in biocatalyst activity. Based on the above results at low ionic strength, the direct immobilization of Pectinex Ultra SP-L, the pH of which is 4.8, on Sepabeads EC-EP was assayed

Figure 11.4 Effect of buffer concentration on the immobilization on Sepabeads EC-EP carriers of transfructosylating activity in Pectinex Ultra SP-L. Immobilization conditions: 30 mg of protein added per gram of support, 24 h, room temperature, and roller shaking. Adapted from Ref. [56].

163

164


without the addition of buffer or salt to control pH and ionic strength. The protein loading (mg) per gram of support was varied over the range 60/1–200/1. Interestingly, the biocatalyst activity reached 25.9 and 18.8 U/g with Sepabeads EC-EP5 and EC-EP3, respectively [56]. Although fructosyl-transfer enzymes have been immobilized by different techniques such as adsorption [71], entrapment [72], or covalent attachment [73], the biocatalyst activity per mass unit is not commonly reported. However, Chiang et al. [74] described a maximum activity of 77 U/g for A. niger β-fructofuranosidase covalently attached to methacrylamide-based polymeric beads. More recently, fructosyltransferase from A. aculeatus present in Pectinex Ultra SP-L was covalently bound to Eupergit C [75]: although the activity recovery was high, the activity per gram of biocatalyst was not reported. 11.4.3 Application of Immobilized Biocatalysts to Fructooligosaccharide Synthesis

Two immobilized biocatalysts, obtained using the direct method on Sepabeads EC-EP3 and EC-EP5, in the transfuctosylation reaction on sucrose were tested. A high substrate concentration (630 g/l) was assayed in order to favor transglycosylation activity. The progress of the reaction with Sepabeads EC-EP3 is depicted in Figure 11.5. It was observed that the fructooligosaccharide concentration reached a maximum value of 387 g/l after 36 h, with a weight ratio of 1-kestose/nystose 1F-fructofuranosylnystose of 62/37/1. At this reaction time, the percentage of fructooligosaccharide in the solids, when referred to the total carbohydrates in the mixture, was 61.5%. After 150 h the mass distribution of the carbohydrates was 57–58% fructooligosaccharide, 29–31% glucose, 9.5–10.5% sucrose, and 2–3% fructose. Similar fructooligosaccharide yields have been reported with other immobilized fructosyltransferases [74, 75].

11.5 Fructooligosaccharide Production Using Sugar Beet Syrup and Molasses 11.5.1 Sugar Beet Syrup and Molasses as Low-cost Feedstock for Fructooligosaccharide Synthesis

Sugar beet syrup is the juice obtained after sugar beet juice evaporation, from which sucrose is crystallized. Molasses is the main by-product of the sugar industry. Both sources are notably less expensive than pure sucrose for production of fructooligosaccharide. The use of molasses for fructooligosaccharide synthesis has only been explored by Shin et al. in cultures of A. pullulans cells [9]. Sugar syrup and molasses are viscous solutions containing sucrose and other carbohydrates (Figure 11.6). Molasses has also more unidentified components and some particu-

11.5 Fructooligosaccharide Production Using Sugar Beet Syrup and Molasses

Figure 11.5 Batch synthesis of fructooligosaccharides catalyzed by immobilized Pectinex Ultra SP-L in Sepabeads EC-EP3. (a) Product distribution and (b) total fructooligosaccharide. Experimental conditions: 630 g/l of sucrose, 0.3 U/ml (standard DNS assay), 50 mM sodium acetate buffer (pH 5.4), and 60 °C. Adapted from Ref. [56].

late materials (especially carbonates). In this work, the sugar syrup and molasses contained 620 and 570 g/l sucrose, respectively. The initial pHs of the sugar syrup and molasses were 7.5 and 8.9, respectively. It was observed that fructooligosaccharide production from syrup and molasses increased substantially when moving from the starting pH to pH 5.5 (by the addition of glacial acetic acid).

165

11 Efficient Fructooligosaccharide Synthesis with a Fructosyltransferase from Aspergillus aculeatus 100

A

Sucrose (74.7 %) Glucose (7.5 %) Fructose (0.7 %) Trisaccharides (13.6 %) Tetrasaccharides (3.5 %)

Sucrose + Betaine

Detector signal (mVolts)

80

60

40 Trisaccharides Glucose to

se

20 Fr

uc

Tetrasaccharides

0 6

9

12

15

18

21

24

27

30

Retention time (min)

50

B

Sucrose (95.7 %) Glucose (2.9 %) Others (1.4 %)

Sucrose + Betaine

40 Detector signal (mVolts)

166

30

20

10

Glucose Other sugars

0

6

9

12

15

18

21

Retention time (min)

Figure 11.6 HPLC chromatograms of (a) the beet sugar syrup and (b) the molasses used in this work. Inset: carbohydrate distribution (in weight).

24

27

30

11.5 Fructooligosaccharide Production Using Sugar Beet Syrup and Molasses

11.5.2 Batch Production of Fructooligosaccharide

In order to reduce the viscosity of the feedstock and to have efficient mixing, several dilutions from syrup and molasses were prepared by adding distilled water to the substrate and adjusting the pH to 5.5. The kinetics of batch fructooligosaccharide production adding 0.2 ml water per milliliter of substrate were analyzed. Table 11.4 summarizes the concentration of total fructooligosaccharide. In the case of the syrup, the fructooligosaccharide concentration reached a maximum value of 387 g/l after 30 h (229 g/l of 1-kestose, 149 g/l of nystose, and 9 g/l of 1Ffructofuranosylnystose). At this reaction time, the percentage of fructooligosaccharide was 56.0%, when referred to the total carbohydrates in the mixture. With regard to the molasses, the maximum fructooligosaccharide concentration was 235 g/l (62 g/l of 1-kestose, 143 g/l of nystose, and 30 g/l of 1F-fructofuranosylnystose) after 65 h, which corresponded to 49.2% fructooligosaccharide. After 140 h the reactions were close to equilibrium, with fructooligosaccharide percentages of 49 and 42% for syrup and molasses, respectively. The results are comparable to those using pure sucrose as the substrate with soluble and immobilized Pectinex Ultra SP-L [56]. As syrup and molasses are colored materials and the fructooligosaccharide specifications for human nutrition are very restrictive in this regard, the fructooligosaccharide obtained by the methods described above could be employed for animal feed. It is well known that the administration of fructooligosaccharides to the diets of some animals results in an improvement in feed efficiency and a reduction in diarrhea and smell in feces [76]. In order to extend the use of

Table 11.4 Fructooligosaccharide production using beet sugar syrup and molasses.

Reaction time (h)

7 24 30 65 115 140

Fructooligosaccharide Syrup (g/l) (%)

Molasses (g/l) (%)

253.1 382.9 387.6 357.9 347.8 341.7

96.2 190.1 223.7 235.5 223.2 202.2

(36.6%) (55.3%) (56.0%) (51.7%) (50.2%) (49.3%)

(20.1%) (39.8%) (46.8%) (49.3%) (46.7%) (42.3%)

Experimental conditions: pH 5.5, 5.0 U/ml (standard DNS assay), 60 °C. The substrate was diluted by adding 0.2 ml of water per milliliter of substrate. Data adapted from Ref. [32] The fructooligosaccharide percentages refer to the total amount of carbohydrates in the mixture.

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fructooligosaccharides as animal feed additives, it is necessary to minimize production costs. Sugar beet syrup and molasses are cheap and available sources of sucrose and are adequate feedstock for fructooligosaccharide production of animalfeed grade.

11.6 Conclusions

The fructosyltransferase mined from the commercial preparation Pectinex Ultra SP-L is quite stable towards pH, temperature, and the presence of chemicals. The A aculeatus fructosyltransferase showed a high transferase : hydrolase ratio that provides it with great potential for oligosaccharide synthesis. The enzyme can be easily immobilized on epoxy-activated supports for better performance. Further studies are required to draw mechanistic conclusions on the nature of the kinetics observed with this fructosyltransferase.

Acknowledgments

The authors thank Drs Moreno Daminati and Paolo Caimi (Resindion S.R.L., Milan, Italy) for providing us with the Sepabeads EC-EP polymers and for technical help. The authors are grateful to Ramiro Martínez (Novozymes A/S, Spain) for providing us with the Pectinex Ultra SP-L. Project BIO2004-03773-C04-01 from the Spanish Ministry of Education and Science and a grant from Genoma España, the National Foundation for Promoting Genomics and Proteomics, supported this research. The authors are also grateful to the Spanish Agency for International Cooperation for a research fellowship.

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(eds H.J. Gilbert, G. Davies, B. Henrissat, B. Svensson), The Royal Society of Chemistry, Cambridge, pp. 3–12. Feng, H.Y., Drone, J., Hoffmann, L., Tran, V., Tellier, C., Rabiller, C. and Dion, M. (2005) The Journal of Biological Chemistry, 280, 37088–37097. Antosova, M. and Polakovic, M. (2001) Chemical Papers-Chemicke Zvesti, 55, 350–358. Sangeetha, P.T., Ramesh, M.N. and Prapulla, S.G. (2005) Process Biochemistry, 40, 1085–1088. Shin, H.T., Baig, S.Y., Lee, S.W., Suh, D.S., Kwon, S.T., Lim, Y.B. and Lee, J.H.

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12 Hydantoin Racemase: the Key Enzyme for the Production of Optically Pure α-Amino Acids Francisco Javier Las Heras-Vázquez, Josefa María Clemente-Jiménez, Sergio Martínez-Rodríguez and Felipe Rodríguez-Vico

12.1 Introduction

Optically pure D-amino acids are valuable intermediates for the preparation of semi-synthetic antibiotics, pesticides, and other products of interest for the pharmaceutical, food, and agrochemical industries [1, 2]. These compounds are named non-natural amino acids and are becoming increasingly important. Typical examples of D-amino acids are D-(phenyl or substituted phenyl) glycines as intermediates of antibiotics such as semi-synthetic penicillins and cephalosporins, D-valine for the synthesis of the insecticide fluvanilate, D-alanine for the production of the dietetic sweetening agent alitame, or D-citrulline and D-homocitrulline used in the potent luterizing hormone releasing hormone (LH-RH) antagonists. There are two main routes for the production of D-amino acids: chemical synthesis and enzymatic catalysis. As regards conventional chemical synthesis, unless asymmetrical starting compounds or catalysts are used, a mixture of the D- and Lstereoisomers is obtained in equal proportions. The racemic mixture is therefore optically inactive and the stereoisomers must be separated. The separation of the enantiomers by classical crystallization of diastereomeric salts is the most costly production step and in any case this method can only yield 50% of the desired enantiomer [3]. Enzymatic synthesis can solve the above problems, providing optical purity of the D-amino acid and a 100% yield, as well as mild, environmentfriendly reaction conditions. One of the most widely used enzymatic methods for D-amino acid production is the ‘hydantoinase process’ [4]. The great advantage of this process is that, potentially, any optically pure D-amino acid can be obtained using the corresponding substrate from a wide spectrum of D,L-5-monosubstituted hydantoins, which are readily accessible by chemical synthesis [5]. In this cascade of reactions the chemically synthesized D,L-5-monosubstituted hydantoin ring is first hydrolyzed by a stereoselective hydantoinase enzyme (D-hydantoinase). Further hydrolysis of the resulting N-carbamoyl D-amino acid to the free D-amino acid is catalyzed


174

12 Hydantoin Racemase: the Key Enzyme for the Production of Optically Pure α-Amino Acids

Figure 12.1 Reaction mechanism of the 5-monosubstituted hydantoins keto-enol tautomerism under alkaline conditions.

Table 12.1 Racemization rate constants (krac) and corresponding half-life times (t1/2,rac) for various hydantoins [7].

5-Substituted hydantoins

Corresponding amino acid

krac (h−1)

t1/2,rac (h)

p-Hydroxyphenylhydantoin Phenylhydantoin Hydroxymethylhydantoin Phenylethylhydantoin Benzylhydantoin Methylthioethylhydantoin Neopentylhydantoin 1-Hydroxyethylhydantoin Trimethylsilylmethylhydantoin 3′-Ureidopropylhydantoin 1′-Methylethylhydantoin Imidazolylmethylhydantoin Isobutylhydantoin Methylhydantoin Ethylhydantoin Isopropylhydantoin

p-Hydroxy-phenylglycine Phenylglycine Serine Homo-phenylalanine Phenylalanine Methionine γγ-Methyl leucine allo-Threonine L-3-Trimethylsilyl-alanine Citrulline allo-Isoleucine Histidine Leucine Alanine 2-Aminobutyric acid Valine

2.26 2.59 0.43 0.144 0.14 0.12 0.11 0.067 0.049 0.044 0.043 0.032 0.020 0.012 0.018 0.0108

0.12 0.27 1.60 4.8 5.00 5.82 6.4 6.41 10.4 14.26 15.84 16.09 21.42 33.98 38.51 55.90

by a highly enantiospecific N-carbamoyl D-amino acid amidohydrolase (Dcarbamoylase). Concurrent to hydantoinase hydrolysis of the D-configurated 5monosubstituted hydantoin, the L-enantiomer is depleted by chemical and/or enzymatic racemization. Chemical racemization of the 5-monosubstituted hydantoins proceeds via ketoenol tautomerism under alkaline conditions, as shown in Figure 12.1 [6]. The racemization velocity is highly dependent on the bulkiness and electronic factors of the substituent in 5-position (see Table 12.1) and is usually a very slow process [7]. High velocities of chemical racemization have only been observed for D,L-5phenyl- and D,L-5-p-hydroxy-phenylhydantoin, because of the resonance stabilization by the 5-substituent, while all other hydantoins take many hours to racemize [8]. Consequently, although the hydantoinase process has been successfully used for producing 100% D-phenyl- and D-p-hydroxy-phenylglycine [9], only 50% of the

12.2 Search for New Hydantoin Racemases and Molecular Characterization

Figure 12.2 Reaction scheme for the hydrolysis of D,L-5monosubstituted hydantoin derivatives to the corresponding D-amino acids.

remaining D,L-5-monosubstituted hydantoins are converted to the corresponding D-amino acid, while the other 50% correspond to L-hydantoin, which is not hydrolyzed by D-hydantoinase. For these hydantoins a faster racemization is possible via an enzymatic equilibration reaction incorporating a third enzyme named hydantoin racemase, in the presence of D-hydantoinase and D-carbamoylase (Figure 12.2). This enzymatic racemization of those D,L-5-monosubstituted hydantoins having a very slow chemical racemization velocity has been shown in several microorganisms by total conversion and 100% production of optically pure Damino acids [10–12], suggesting that a hydantoin racemase might be responsible for the fast racemization of the substrates. In this way, complete and fast racemization is obtained, the velocity of hydrolysis is increased, and accumulation of one isomer (L-hydantoin) is avoided (Figure 12.3).


Given the important role that hydantoin racemase plays in the production of optically pure D-amino acids, allowing the racemization of 5-monosubstituted

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Figure 12.3 Reaction profile of D-methionine production from D,L-methylthioethylhydantoin (D,L-MTEH) using (a) a double system with D-hydantoinase and D-carbamoylase enzymes and (b) a triple system with hydantoin racemase enzyme as third enzyme. Symbols: 䊉, D-methionine; 䊊, N-carbamoyl Dmethionine; ⵧ, D,L-methylthioethylhydantoin; 䊏, sum of all three [9].

hydantoins under unfavorable physiological conditions for chemical racemization, our laboratory started the search for such enzymes in different sources. Knowledge of the biochemical, physical, and thermodynamic characteristics of hydantoin racemase enzymes will allow a better design of biocatalysts, such as immobilized enzymes or whole cell biocatalysts. Previous works have studied two hydantoin racemases from both the molecular and biochemical angles, but these were


involved in the production of L-amino acids [13–16]. As for the presence of hydantoin racemase involved in optically pure D-amino acid production, a putative hydantoin racemase was described together with D-hydantoinase and Dcarbamoylase in a DNA fragment from Agrobacterium sp. IP-I 671 [17]. However, from the biochemical and enzymatic points of view there was no information on hydantoin racemases involved in the production of D-amino acids. In 2001 two different genome sequences of Agrobacterium tumefaciens C58 and Sinorhizobium meliloti were reported [18–20]. In the linear chromosome of each genome, together with other protein-coding genes, our laboratory found two putative hydantoin racemases with high sequence homology to previously described hydantoin racemases. Four fragments were amplified by a polymerase chain reaction and cloned in expression plasmids. While both sequences from A. tumefaciens showed hydantoin racemase activity in the recombinant cells, only one was active in S. meliloti. Recently a hydantoin racemase from Microbacterium liquefaciens AJ 3912 has also been purified [21]. These six hydantoin racemases are the only ones isolated and studied to date and they show a high homology in their amino acid sequences (Figure 12.4), with identity percentages of over 40%. This increases to 80% when comparing hydantoin racemase 1 from A. tumefaciens C58 (AtHyuA1) [9, 22] and the one from S. meliloti CECT 4114 (SmeHyuA) [23] or those from Arthrobacter aurescens DSM 3747 (AauHyuA) [15] and from M. liquefaciens AJ 3912 (MliHyuA) [24]. All six hydantoin racemases have shown an amino acid sequence length of between 236 and 247 residues. The phylogenetic tree generated by comparing the complete amino acid sequences of hydantoin racemases revealed the existence of three branches (Figure 12.5): one comprises AtHyuA1 and SmeHyuA, a second branch comprise AauHyuA and MliHyuA, together with hydantoin racemase from Pseudomonas sp. NS671 (PspHyuE) [13], and the third branch comprises only hydantoin racemase 2 from A. tumefaciens C58 (AtHyuA2) [25]. The genetic organization of the enzymes involved in the hydantoinase process (hyu), including hydantoin racemase (hyuA), hydantoinase (hyuH) and Ncarbamoylase (hyuC) genes has been reported. There are two clearly different distributions of the genes, depending on whether the hyu gene cluster is involved in L- or D-amino acid production (Figure 12.6). Thus, two of the three L-specific hyu gene clusters described corresponding to A. aurescens DSM 3747 [26] and M. liquefacines AJ 3912 [24] are very similar. These two hyu gene clusters and that of Pseudomonas sp. NS671 [13] have the same orientation and it was suggested that they constitute an operon, harboring a putative hydantoin transporter gene in addition to hydantoin racemase, L-N-carbamoylase and L-hydantoinase genes. On the other hand, the non-selective hydantoinase from Pseudomonas sp. NS671 consists of two subunits, hyuA and hyuB. It shows similarity to Nmethylhydantoinases, which belong to the superfamily of ATP-dependent cyclic amidases [27]. However, in the D-specific hyu gene cluster D-N-carbamoylase and D-hydantoinase genes are transcribed from a common intercistronic promoter region in opposite orientation [17].

177

Figure 12.4 Multiple alignment of the amino acid sequences of hydantoin racemases. Hydantoin racemase 1 from A. tumefaciens C58 (AtHyuA1), GenBank accession no. AY436503; hydantoin racemase from S. meliloti CECT 4114 (SmeHyuA), GenBank accession no. AY393697; hydantoin racemase 2 from A. tumefaciens C58 (AtHyuA2),

GenBank accession no. AY436504; hydantoin racemase from A. aurescens DSM 3747 (AauHyuA), GenBank accession no. AF146701; hydantoin racemase from M. liquefaciens AJ 3912 (MliHyuA), GenBank accession no. BD181023; hydantoin racemase from Pseudomonas sp. NS671 (PspHyuE), GenBank accession no. M84731.

178



Figure 12.5 Phylogenetic analysis of the amino acid sequences of hydantoin racemases characterized to date. For abbreviations of the enzymes, see the caption to Figure 12.4.

Figure 12.6 Genetic organization of the hyu gene clusters from Pseudomonas sp. NS671, A. aurescens DSM 3747, M. liquefaciens AJ 3912 and A. tumefaciens IPI-671. Genes and their transcriptional orientation are symbolized by arrows. Genes encoding related proteins are represented by arrows with identical patterns.

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12.3 Biochemical Characterization of Hydantoin Racemase Enzymes

The apparent molecular mass of the subunit of the different hydantoin racemases is very similar (Table 12.2) and larger than that calculated from the amino acid sequence, with values of 25 000–27 000 Da. These differences can be explained by a ‘molten globule’ state, where the enzymes in sodium dodecyl sulfate– polyacrylamide electrophoresis gel (SDS–PAGE) conditions are compact denatured structures but with near native secondary structure, significant loss of tertiary structure, and increased exposure of hydrophobic surface area. In SDS–PAGE these partially denatured hydantoin racemases moved slower than if they were totally denatured. The relative molecular mass of the native enzymes measured by size exclusion chromatography (high-performance liquid chromatography or HPLC) has been estimated at 100 000 Da, with the exception of that reported for Pseudomonas sp. (190 000 Da) and for A. aurescens DSM 3747 (175 000 Da). Thus, a tetrameric structure is deduced for hydantoin racemase enzymes from Agrobacterium, Sinorhizobium, and Microbacterium, whereas the Pseudomonas sp. hydantoin racemase enzyme has been described as hexameric and the A. aurescens DSM 3747 hydantoin racemase enzyme as hexameric, heptameric, or octameric (Table 12.2). Although on the whole the hydantoin racemases have shown high thermal stability, with optimal activity at 55 °C (Table 12.2), the optimal temperatures for the ones from Pseudomonas and Sinorhizobium decrease to 45 and 40 °C, respectively. However, the optimal pH is higher than 8, except for both hydantoin racemases from Agrobacterium. This low alkaline pH avoids chemical racemization. Consequently, the racemization of the D- or L-5-monosubstituted hydantoins in an industrial process will only occur enzymatically. The activity of the purified hydantoin racemase enzymes has been assayed in the presence of different metal ions, the chelating agent EDTA, and the reducing compound dithiothreitol, in order to study whether they are metalloenzymes. Most

Table 12.2 Biochemical properties of hydantoin racemases described to date.

Strain

System chirality

Molecular weight of Monomers (kDa)

Number of subunits

Optimal pH

Optimal temperature (°C)

Pseudomonas sp. NS671 A. aurescens DSM 3747 A. tumefaciens C58 1 A. tumefaciens C58 2 M. liquefaciens AJ 3912 S. meliloti CECT 4114

L

32 32.1 32 27 27 31

6 6, 7, and 8 4 4 4 4

9.5 8.5 7.5 7.5 8.2 8.5

45 55 55 55 55 40

L D D L D

12.4 Substrate Enantioselectivity and Kinetic Analysis of Hydantoin Racemases

of the metal ions had no significant effect on the enzymes and an inhibitory effect was only detected in the presence of Cu2+, Hg2+, Pb2+, and Zn2+. No effect was detected after enzyme incubation with dithiothreitol, with the exception of the Arthrobacter enzyme, which was stimulated. However, the insignificant effect of the metal chelating agent EDTA on the activity of the hydantoin racemases would indicate that they are not metalloenzymes.

12.4 Substrate Enantioselectivity and Kinetic Analysis of Hydantoin Racemases

The natural substrates for hydantoin racemases are as yet unknown, but for industrial purposes their ability for racemizing different 5-monosubstituted hydantoins with very low chemical racemization has been studied (Figure 12.7). Of the six hydantoin racemases, only the Arthrobacter one has shown better racemization for hydantoins with aromatic substituents, the enzyme from Microbacterium was only studied with one substrate and the other enzymes racemized the aliphatic substrates faster (see the example of AtHyuA1 in Figure 12.8). For Agrobacterium hydantoin racemases 1 and 2 a reciprocal dependence between kinetic activity and the substituent chain length was detected for hydantoins with aliphatic substituents. The highest racemization velocity corresponded to the shortest length in the substituent chain. However, racemization of D- and L-hydantoins took longer with AtHyuA2 than with AtHyuA1. This faster racemization by AtHyuA1 is corrobo-

Figure 12.7 Reaction profile of the enzymatic racemization of D- and L-5-monosubstituted hydantoin to the racemic mixture by hydantoin racemase enzyme monitored by chiral HPLC (see the methodology in [28]).

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Figure 12.8 Enzymatic racemization of the D-isomer (䊉) and L-isomer (䊊) of (a) 5-benzylhydantoin and (b) 5ethylhydantoin by hydantoin racemase 1 from A. tumefaciens C58 (AtHyuA1).

rated by more favorable kinetic parameters, with lower Km and higher kcat values than for AtHyuA2 (Table 12.3). A singular effect of inhibition by 5-monosubstituted hydantoins has been detected for hydantoin racemase from Sinorhizobium. The D-isomer of the potential substrate 5-ethylhydantoin had no detectable racemization under standard conditions, while its L-isomer (L-5-ethylhydantoin) showed the fastest racemization of all hydantoins analyzed. On the other hand, neither the D- nor the L-isomer of 5-methylthioethylhydantoin showed any racemization in the presence of the Sinorhizobium hydantoin racemase enzyme (Figure 12.9). These substrates towards which the enzyme had no detectable activity were studied as possible inhibitors of the hydantoin racemase. No inhibition effect was detected by D-5ethylhydantoin for the substrate L-5-ethylhydantoin. However, inhibition by Dand L-5-methylthioethylhydantoin was studied at varying concentrations of L-5-ethylhydantoin. Double-reciprocal plots of the data indicated that both D- and L-5-methylthioethylhydantoin are competitive inhibitors of L-5-ethylhydantoin as the variable substrate (Figure 12.10). Substrate inhibition processes have been described for Pseudomonas sp. NS671 and A. aurescens DSM 3747 hydantoin racemases, even at low substrate concentrations. However, this phenomenon has not been detected either for Agrobacterium hydantoin racemase. Hydantoin racemase from Sinorhizobium has shown no activity with D- and L-5-methylthioethylhydantoin substrate as was previously reported for Pseudomonas sp. NS671 hydantoin racemase towards either isomer of 5-isopropylhydantoin. However, this is the only case in which substrates that the enzyme is not capable of racemizing have been studied as inhibitors of hydantoin racemase activity.

12.5 Proposal for a Reaction Mechanism of Hydantoin Racemase Enzymes Table 12.3 Kinetic parameters of A. tumefaciens C58 hydantoin racemase 1 and 2 (ATHyuA1 and AtHyuA2).

Substratea O

H N

L-EH

H N

D-EH

O

H N

kcat/Km (s−1mM−1)

AtHyuA1 AtHyuA2

4.45 ± 0.060 19.42 ± 2.34

13.64 ± 1.53 1.81 ± 0.01

3.06 ± 0.63 0.09 ± 0.01

AtHyuA1 AtHyuA2

4.26 ± 0.70 12.54 ± 1.81

16.47 ± 1.47 1.80 ± 0.10

3.87 ± 0.59 0.14 ± 0.03

O

H N

AtHyuA1 AtHyuA2

5.56 ± 1.45 18.42 ± 4.80

1.07 ± 0.23 0.18 ± 0.02

0.19 ± 0.04 0.01 ± 0.00

H N

AtHyuA1 AtHyuA2

4.71 ± 0.62 20.77 ± 5.47

2.36 ± 0.43 0.46 ± 0.07

0.50 ± 0.10 0.02 ± 0.00

H N

AtHyuA1 AtHyuA2

5.41 ± 1.06 1.90 ± 1.48

1.57 ± 0.37 0.78 ± 0.07

0.35 ± 0.08 0.07 ± 0.01

H N

AtHyuA1 AtHyuA2

4.47 ± 0.96 6.31 ± 0.31

2.03 ± 0.45 0.5 ± 0.01

0.45 ± 0.12 0.08 ± 0.01

H N N H

D-IBH

AtHyuA1 AtHyuA2

1.23 ± 0.20 3.02 ± 0.78

2.05 ± 0.53 0.48 ± 0.02

1.66 ± 0.13 0.16 ± 0.05

AtHyuA1 AtHyuA2

4.58 ± 0.51 6.79 ± 0.60

4.16 ± 0.67 0.83 ± 0.03

0.90 ± 0.08 0.12 ± 0.01

O

N H

O

D-MTEH O

N H O

L-MTEH O

N H

O

D-BH O

N H O

L-BH O

N H

S

kcat (s−1)

O

N H

S

Km (mM)

O

N H O

Enzyme

D-IBH O

a D- and L-EH: D- and L-5-ethylhydantoin; D- and L-BH: D- and L-5-benzylhydantoin; D- and LMTEH: D- and L-5-methylthioethylhydantoin; D- and L-IBH: D- and L-5-isobutylhydantoin. These parameters were determined at 40 °C for 15 min at pH 7.5. The kcat was defined as the D- or L-5-monosubstituted hydantoin (mmol) racemized per second and of enzyme (mmol) at 40 °C.

12.5 Proposal for a Reaction Mechanism of Hydantoin Racemase Enzymes

The amino acid sequences of known hydantoin racemases present two highly conserved cysteines around positions 75 and 180 (see the asterisks in Figure 12.4). The enzymes involved in the racemization/epimerization of different substrates such as glutamate racemase and diaminopimelate epimerase present two cyste-

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Figure 12.9 Enzymatic racemization by hydantoin racemase from S. meliloti CECT 4114 of the D-isomer (䊊) and L-isomer (䊉) of 5-methylthioethylhydantoin. Chemical racemization of the D-isomer (䉮) and L-isomer (䉲) of each substrate was also measured at the same intervals.

ines involved in the catalytic center. To demonstrate the role of these two cysteines in the enzyme activity, our group has carried out structural, kinetic, and binding studies showing the importance of these residues in the recognition and racemization of the substrates by hydantoin racemase enzyme [29, 30]. For these studies hydantoin racemase from S. meliloti was selected. Comparison of the amino acid sequence of SmeHyuA with those of aspartate or glutamate racemases of well-known activity revealed low similarity percentages. However, two completely conserved cysteines have been shown to be involved in the racemization process for glutamate and aspartate racemases and, thus, belong to the active site. In order to study the role of the two cysteines of hydantoin racemase in racemization, our laboratory developed mutants of these cysteines in Sinorizhobium. Thus, cysteine 76 and cysteine 181 were mutated to serine and alanine, thereby obtaining four mutants (C76S, C76A, C181S, and C181A). Activity assays with mutants show a drastic decrease in activity of the enzyme, indicating that both cysteine 76 and 181 are essential for catalysis. Substitution by a residue able to transfer a proton (serine) retained a small fraction of the activity, whereas substitution by an alanine resulted in no detectable activity, thus indicating that the presence of a proton donor group in that position is critical for the catalysis of this enzyme. Similar behavior has been detected in other enzymes acting by a two-base mechanism. The binding affinities of SmeHyuA C76A and C181A active site mutants (alanine mutants) were evaluated for further insight into the role of each residue. Binding of the above substrates to both mutants was also measured by isothermal titration calorimetry which showed that the presence of a proton at the 5-position of the substrate and not in the lateral chain is the critical factor for proper binding.

12.5 Proposal for a Reaction Mechanism of Hydantoin Racemase Enzymes

Figure 12.10 (a) D-5-Methylthioethylhydantoin (D-MTEH) and (b) L-5methylthioethylhydantoin (L-MTEH) inhibition patterns with L-5-ethylhydantoin (L-EH) as the variable substrate. The D- and L-MTEH concentrations were varied in the presence of a constant concentration of L-EH: (䊉) no MTEH added, (䊊) 3 mM MTEH, (䉲) 6 mM MTEH and (䉮) 9 mM MTEH. Sinorhizobium

meliloti hydantoin racemase activity was assayed at 40 °C for 15 min at pH 8.5. The insets show the plot of the apparent Km/Vmax ((Km/Vmax)app) versus the inhibitor concentration (D- or L-MTEH), giving Ki values of 5.75 and 3.76 mM for (a) and (b), respectively. Reactions were performed in triplicate and yielded kinetic constants with less than 10% standard error from the mean.

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Additional binding experiments conducted by fluorescence measurements with C76A mutant and D- and L-isopropylhydantoin and L-ethylhydantoin (Dethylhydantoin is an inhibitor) showed that this mutant is unable to bind the D-isomers of the substrates. The same experiments carried out with C181A mutant proved that this mutant was not able to bind the L-isomers. These results indicate that cysteine 76 is responsible for the recognition of D-isomers of the 5monosubstituted hydantoins, whereas cysteine 181 recognizes the L-isomers. To confirm these results we also analyzed the stability of the mutant by unfolding studies with guanidinium hydrochloride as denaturant. The addition of Disopropylhydantoin to C76A or L-isopropylhydantoin to C181A increased the stability of the active site mutants when incubated in the presence of guanidinium hydrochloride. These experiments again suggest that cysteine 76 is a key residue for the recognition of D-isomers of the 5-monosubstituted hydantoins, whereas cysteine 181 is indispensable for the recognition of L-isomers. In order to ascertain how these two cysteines are arranged in the catalytic center, a three-dimensional homology model for SmeHuyA was developed using other previously resolved racemases, such as aspartate racemase from Pyrococcus and glutamate racemase from Aquifex (Figure 12.11) [30]. This model supported the hypothesis that cysteines 76 and 181 are located opposite one another and further confirmed the ‘twobase mechanism’ proposed for SmeHyuA. Moreover, this catalytic behavior was inferred from the observations in a previous work with hydantoin racemase from A. aurescens [15], where reactions followed by nuclear magnetic resonance in D2O showed that the solvent isotope was efficiently incorporated into the product enantiomer, but not into the substrate enantiomer, regardless of which one served as substrate. From these results, the deduced model is that of a two-base mechanism (Figure 12.12). In this model, when a D-isomer of a 5-monosubstituted hydantoin is

Figure 12.11 Superposition of active sites from the different proteins. The active site of SmeHyuA model (gray) is compared with that of (a) aspartate racemase from Pyrococcus horikoshii (1FJLA, black) and that of (b) glutamate racemase from Aquifex pyrophilus (1B73A, black).

12.6 Design of a Tailormade Recombinant Biocatalyst Including Hydantoin Racemase Enzymes

Figure 12.12 Proposed racemization mechanism of hydantoin racemase using a two-base mechanism. The hypothetical character of the intermediate structure is indicated by the dashed lines surrounding the figure.

available, cysteine 76 (in a thiolate form) would act as a base and retrieve a proton. A plane intermediate of the substrate would be formed, as has been proposed for the chemical racemization of the hydantoins [8] and for the racemization of D- and L-glutamate by glutamate racemase. Cysteine 181 would act as an acid adding a proton to the opposite side of the substrate, thus producing the L-5monosubstituted hydantoin. On the other hand, the binding to cysteine 181 and the retrieval of a proton would carry out the isomerization of the L-isomer of the substrate. In this case, this residue would act as the base and cysteine 76 would donate a proton to the putative intermediate formed.

12.6 Design of a Tailormade Recombinant Biocatalyst Including Hydantoin Racemase Enzymes for Optically Pure D-Amino Acid Production

Once several hydantoin racemases had been studied and characterized, their behavior, together with the other enzymes involved in the hydantoinase process,

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188


was analyzed with a view to producing optically pure D-amino acids. Our group developed a three-step enzymatic reaction for D-amino acid production using a recombinant whole cell biocatalyst after separate expression in three different Escherichia coli strains of D-hydantoinase, D-carbamoylase, and hydantoin racemase from Agrobacterium [9]. However, enzyme production requires three individual cultivations and transport of the reaction intermediate might be a limiting factor. Alternatively, the three enzymes can be co-expressed in the same host cell and directly used as biocatalysts. The first recombinant system co-expressing these three genes in E. coli was employed to produce L-amino acids cloning hydantoin racemase together with L-hydantoinase and L-carbamoylase, all from A. aurescens [16]. More recently, D-hydantoinase and D-carbamoylase from Flavobacterium sp. and hydantoin racemase from M. liquefaciens have been co-expressed in E. coli [31]. These two systems co-express the three genes after cloning in plasmids with different antibiotic resistance genes. This strategy involves adding several antibiotics to the medium, resulting in high selective pressure in the growth of the recombinant cells. In addition, both of these recombinant systems have shown intermediate accumulation as a result of weak D-carbamoylase activity, which has been described as a limiting factor in the overall process [32]. In order to overcome these two problems our group designed a biocatalyst for the production of optically pure D-amino acids from D,L-5-monosubstituted hydantoins by co-expressing the three genes in one plasmid in a polycistronic structure [33]. Two whole cell recombinant systems were designed, both of which contain two common genes, D-hydantoinase and D-carbamoylase from A. tumefaciens BQL9. The third gene was hydantoin racemase 1 (AthyuA1) [22] for the first system and hydantoin racemase 2 (AthyuA2) [25] for the second one, both from A. tumefaciens C58. The induction conditions of the polycistronic structures were optimized and several E. coli strains were analyzed as hosts for the recombinant plasmids. Both constructions showed the highest activity after 8 h induction in the culture at 34 °C. However, the optimal inductor concentration was different, 0.1 mM of isopropyl-beta-D-thiogalactopyranoside (IPTG) for system 1 and 0.2 mM for system 2. Four E. coli strains (DH5α, JM109, TOP10F, and BL21) were studied as hosts for the recombinant systems and E. coli strain BL21 proved to be the best for both systems. Stability studies of both recombinant plasmids in this strain have demonstrated that the recombinant cells have grown for at least 90 generations without ampicillin pressure after the initial inoculation. The optimum temperature for both whole cell systems was around 55 °C, reaching 100% conversion from D,L-methylthioethylhydantoin to D-methionine at pH 8. System 1 was able to hydrolyze all the substrate D,L-methylthioethylhydantoin and achieve total conversion to obtain optically pure D-methionine in about 100 min (Figure 12.13a). System 2 was slower, taking about 200 min to attain 100% conversion of the substrate to the product (Figure 12.13b). Other hydantoins, with both aliphatic and aromatic substituents in carbon 5, were hydrolyzed to the corresponding optically pure D-amino acid (Figure 12.14). All the substrates studied, whether of slow or fast chemical racemization, were converted by both systems to the corresponding D-amino acids without accumulation of the intermediate


Figure 12.13 Comparison of the reaction profile of Dmethionine (D-met) production from 15 mM D,Lmethylthioethylhydantoin (D,L-MTEH) without D-carbamoyl-methionine (D-car-met) accumulation using (a) system 1 and (b) system 2.

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190


Figure 12.14 Initial reaction rate for the production of different optically pure D-amino acids from 5-monosubstituted hydantoins using systems 1 and 2. TRP, D-tryptophan; TYR, D-tyrosine; pHPG, D-p-hydroxyphenyl glycine; VAL, D-valine; PG, D-phenylglycine; ABA, D-aminobutyric acid; PA, Dphenylalanine; NVA, D-norvaline; NLEU, D-norleucine; MET, D-methionine; LEU, D-leucine. Reactions were performed in triplicate and error bars represent the standard error from the mean.

N-carbamoyl-D-amino acid. This is the first time that this bottleneck has been overcome. There was no substrate inhibition due to the higher overexpression of D-carbamoylase compared to the other two enzymes involved in this biotransformation. Both systems showed higher reaction rates for the conversion of aliphatic amino acids than for aromatic ones. System 1 was able to hydrolyze the 5-monosubstituted hydantoins faster than system 2 for the production of almost all the D-amino acids studied. System 1 was slightly slower than system 2 only for the production of the aromatic amino acids D-tyrosine and D-phenylglycine. This agrees with previously described results, finding that AtHyuA1 enzyme (included in system 1) was more viable for industrial application than AtHyuA2 (included in system 2) due to its higher substrate affinity and racemization velocity [25]. The behavior of the recombinant biocatalyst on a larger scale is of considerable economic interest. For this reason D-methionine production was analyzed from 300 mM D,L-methylthioethylhydantoin (52.3 g/l), in a reaction volume 300 times that used at the laboratory scale (300 ml). When system 1 was induced in optimal conditions the product yield of D-methionine reached 100% in 6 h and there was no D-carbamoyl-methionine accumulation (Figure 12.15a). However, when the


Figure 12.15 Profile of D-methionine (D-met) production from 300 mM D,L-methylthioethylhydantoin (D,L-MTEH) without Dcarbamoyl-methionine (D-car-met) accumulation using system 1 (a) induced with 0.1 mM IPTG and (b) non-induced in a large-scale reaction with a final volume of 300 ml. Reactions and measurements were carried out in triplicate and error bars represent the standard error from the mean.

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192


inducer IPTG was not added, this system showed less activity, reaching total conversion at approximately 32 h (Figure 12.15b). The development of this multienzymatic system for the production of D-amino acids from any D,L-5-monosubstituted hydantoin allows the hydantoinase process to produce not only two amino acids, such as D-phenylglycine and D-phydroxy-phenylglycine (as explained at the beginning of the chapter), but also many non-natural D-amino acids that could be components of potential pharmaceuticals.

Acknowledgments

The authors gratefully acknowledge funding of this research by grants from the Ministerio de Educacion y Ciencia, Spain (BIO 2007-67009) and from Consejeria de Innovacion, Ciencia y Tecnologia, Andalusia region, Spain (P07-CVI-2651).

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13 Chemo-enzymatic Deracemization Methods Davide Tessaro, Gianluca Molla, Loredano Pollegioni and Stefano Servi

13.1 Introduction

In recent times, the term deracemization has been used for describing techniques becoming increasingly important for the preparation of chiral compounds as single enantiomers. The term refers to a process in which one single enantiomer is obtained when starting from a racemate [1]. The two more common strategies for achieving such an objective are deracemization by stereoinversion or deracemization by dynamic kinetic resolution DKR (Scheme 13.1). Deracemization by stereoinversion is a process in which one form (Sf) of the racemic starting material (Rf + Sf) is enantioselectively transformed into an intermediate (Si) which can in turn react to give the form of opposite configuration (Rf). An example of this method could be the selective oxidation of one enantiomer of a racemic secondary alcohol and the subsequent reduction with a catalyst of opposite stereopreference [2]. Deracemization by DKR is in principle a kinetic resolution process in which the non-transformed enantiomer is racemized in situ. The conditions are that a chiral catalyst promotes the transformation of one enantiomer (Rs) into the product (Rp) while the other enantiomer is racemized at a comparable rate and the racemic mixture (Rs + Ss) is restored. The product (Rp) is not racemized under the same conditions. While a simple kinetic resolution yields a maximum of 50% of the product, with this technique a 100% conversion can be reached. Although the majority of chiral molecules of industrial interest are still prepared by kinetic resolution, the continuous development of industrial enzymes and racemizing processes fosters new chemo- or biocatalytic systems for DKR to appear. A great impulse for deracemization methods based on the one-pot/two-steps resolution racemization process was brought about by Bäckvall et al. over a 10-year period [3]. The method is of general applicability in the deracemization of secondary alcohols and amines and consists of a lipase-catalyzed irreversible acylation and in situ racemization of the non-reacted enantiomer catalyzed by a ruthenium catalyst, Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

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13 Chemo-enzymatic Deracemization Methods

Scheme 13.1 Deracemization by stereoinversion and by DKR.

usually in an amount of 2 mol %. The two steps occur in the same medium, an organic solvent. Many other deracemization methods based on a two-enzyme system or in the successive enzymatic resolution/base-catalyzed racemization have been reported. They are often regarded as having a reduced environmental impact, not requiring a transition metal catalyst in the racemization step [4]. Unlike a kinetic resolution process, where in principle both enantiomers of the same compound can be obtained, a deracemization process gives access to one single enantiomer. The possibility of securing the enantiomer of opposite configuration depends on the availability of the enzymes involved: frequently enzymes with complementary stereopreferences are not available. In this chapter deracemization methods for the preparation of multifunctional compounds (α- and β-hydroxy acids, α-hydroxynitriles, and α-amino acids) are discussed (Scheme 13.1).

13.2 Deracemization Methods for α- and β-Hydroxy Acids

Many α- and β-hydroxy acids are found as natural products or incorporated into bioactive compounds and are required in both absolute configurations [5]. The biocatalytic asymmetric synthesis of these compounds has been reported in detail [6]. Since the racemic forms are easily available through conventional synthetic methods, the application of deracemization methods appears a convenient approach for obtaining single enantiomers.


13.2.1 Deracemization of Hydroxy Acids by DKR (Hydrolytic Enzymes + Ruthenium-based Racemization Catalysts)

The method of combined enzyme- and transition metal-catalyzed reactions widely applied to the DKR of secondary alcohols has also been applied to the DKR of αhydroxy acid esters rac-1. The principle is based on the enantioselective acylation catalyzed by Pseudomonas species lipase (PS-C from Amano Ltd) using p-Cl-phenyl acetate as an acyl donor in cyclohexane combined with in situ racemization of the non-acylated enantiomer catalyzed by ruthenium compounds [7]. Under these conditions, various α-hydroxy esters of type 1 were deracemized in moderate to good yields and high enantioselectivity (Scheme 13.2). The use of the ruthenium catalysts is advantageous since it does not need the presence of a base as a co-catalyst. The presence of a base could induce racemization of the formed acetates by enolization. The same methodology was also applied to the DKR of β-hydroxyesters. In the latter case, the reaction was carried out in tandem with an aldol reaction and the β-hydroxyester formed, after neutralization, underwent DKR using the immobilized lipase from Candida antarctica and a ruthenium catalyst [8]. The β-acetoxy esters 2 were obtained in yields over the range 69–75%, based on the aldehyde and with a high enantiomer excess (ee). In addition, in this case the absence of a base as a co-catalyst is essential in order to avoid retro-aldol reaction of the product. The scope of this methodology is not restricted to aromatic hydroxy acids, as the non-aromatic cyclohexyl analog was also successfully deracemized (Scheme 13.3).

Scheme 13.2 Deracemization of α−hydroxy acids by DKR (hydrolytic enzymes + ruthenium-based racemization catalyst).

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Scheme 13.3 DKR of in situ-formed β-hydroxyesters coupling a lipase enantioselective irreversible acylation with rutheniumcatalyzed racemization (R = Ph, p-OMe-Ph, PhCH2-).

13.2.2 Deracemization of Hydroxy Acids by DKR with a Two-enzyme System

Deracemization of mandelic acid with the combined action of two enzymes has been reported. rac-Mandelic acid is acylated by a Pseudomonas sp. lipase in diisopropyl ether. After solvent removal the mixture of mandelic acid enriched in the R-form and the O-acetyl derivative of the S-configuration are subjected to the mandelate racemase-catalyzed racemization in aqueous buffer. In these conditions only the non-acetylated hydroxy acid is racemized. In order to obtain (S)-Oacetylmandelic acid in an 80% isolated yield and a >98% e.e. the process must be repeated four times [9]. This example of a classical cyclic resolution–racemization methodology indicates the limitations of a multistep enzyme-catalyzed reaction when the reaction medium, the reagents, or the reaction conditions are not mutually compatible. Mandelate racemase, which is probably the best known of all racemizing enzymes [10], shows an activity limited to compounds structurally related to mandelates [11]. The presence of an aromatic group or an unsaturation in the αposition is essential for activity. In order to extend the two-enzyme system to other 2-hydroxy acids, a racemase with a broader activity was found in Lactobacillus paracasei. This was exploited for deracemization of 2-hydroxy-4-phenylbutanoic acid and 3-phenyllactic acid, which are important synthetic intermediates. In addition, in this case the procedure requires a kinetic resolution step and a successive racemization step. O-Acetyl derivatives of the absolute (S)-configuration can be obtained in two successive repeating cycles. Yields are around 60%. Of course the O-acetyl derivatives of opposite configuration can be obtained when the lipase-catalyzed reaction is applied in the hydrolysis direction. Obtaining the O-acetyl derivatives of the absolute (R)-configuration requires an additional acetylation step of the initially resolved and racemized (S)-hydroxy acid [12].


13.2.3 Deracemization of Hydroxy Acids by Stereoinversion

α-Hydroxy acids of the (R)-configuration can be obtained through deracemization by stereoinversion of the racemic lactate analogs 3 (Scheme 13.4). The deracemization takes place through a selective oxidation catalyzed by L-specific glycolate oxidase followed by a reductive step catalyzed by D-lactate dehydrogenase [13]. In this procedure the NADH co-factor is then regenerated in situ by the formate dehydrogenase/ammonium formate method (Scheme 13.4). The method is similar to the deracemization of α-amino acids (see Section 13.4.1) with a combination of amino acid oxidase and amino transferase or amino acid dehydrogenase. Although the co-factor regeneration is considered a solved problem, the application of the method to obtaining α-hydroxy acids of wide structural variety and of both configurations is also in this case dependent on enzyme availability [13]. Alternatively, a ‘one-pot, single-step’ deracemization of sec-alcohols has been achieved by employing two different microorganisms in a single reaction vessel. However, the number of examples of this type is limited and the oxidation and reduction steps are usually performed sequentially in a one-pot, two-step procedure. For instance, racemic mandelic acid was deracemized in the presence of whole cells of Pseudomonas polycolor and Micrococcus freudenreichii [14]. Separate experiments showed that P. polycolor was responsible for the oxidation, while M. freudenreichii was needed for reduction of the corresponding α-keto acid. After 24 h, (R)-mandelic acid 4 was isolated in a 60% yield and 99% e.e. [14]. The two-enzyme system was also used to convert L-lactate into D-lactate with a yield better than 97%. L-Lactate is oxidized by L-lactate dehydrogenase to give pyruvate. The keto acid is reduced in an electrochemical system at the cathode to racemic lactate and NADH is oxidized to NAD+ at the anode. The continuous

Scheme 13.4 Deracemization of hydroxy acids by stereoinversion.

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Scheme 13.5 Whole cell deracemization by stereoinversion.

Scheme 13.6 Lactate chemoenzymatic deracemization.

oxidation of L-lactate allows the accumulation of the D-enantiomer. The application of the method to deracemization or complete inversion of chiral 2-hydroxy acids requires specific dehydrogenases [15] (Scheme 13.5). In a conceptually related method, the oxidation of the L-lactate is catalyzed by the commercially available L-specific lactate oxidase from Aerococcus viridans and the reduction of pyruvate to rac-lactate is performed with NaBH4 in the same solution. In repeating cycles D-lactate 5 is obtained as the sole product and in an excellent e.e. [16] (Scheme 13.6). This concept has recently been applied to the deracemization of α-amino acids [17]. 13.2.4 Deracemization of Hydroxy Acids by Microbial Stereoinversion

Microbial stereoinversion consists of a deracemization process in which a single whole cell system is applied for the two-step inversion of the configuration of one enantiomer, usually a compound containing a secondary alcohol group. Examples of a two-enzyme system is known for deracemization of mandelates where a single microorganism is used [18]. Recently the microorganism Candida parapsilosis was used for the deracemization of α- and β-hydroxy esters. A series of racemic aryl and aryl substituted

13.3 Deracemization of α-Hydroxy Nitriles

Scheme 13.7 Hydroxy acids produced by microbial stereoinversion.

α-hydroxy esters were deracemized by the whole cells of C. parapsilosis to give the corresponding optically pure (S)-hydroxy esters in good yields (65–85%) and e.e. (90–99%). The mechanism of deracemization involves two different enzymatic reactions: oxidation of one of the antipodes by an (R)-oxidase to the keto intermediate followed by its subsequent reduction by a (S)-specific reductase enzyme [19] (Scheme 13.7). In order to clarify the proposed mechanism, the reduction of ethyl 3-oxo-3phenyl propionate was carried out, resulting in the exclusive formation of the (S)enantiomer confirming the presence of (S)-specific reductase. When the pure (S)-enantiomer was used as the substrate for deracemization, the product recovered retained the configuration as well as the optical purity without any loss, clearly supporting the presence of an (S)-specific reductase, which does not oxidize the (S)-hydroxy ester to the corresponding keto ester. The generality of the reaction was established by using different substituents in the standard substrate ethyl 3-hydroxy 3-phenyl propionate. The presence of electron donating and electron withdrawing groups in the para position does not affect the deracemization reaction, while substitution in the ortho position hinders the same. On a preparative scale, up to 500 mg of racemic substrate incubated with the cells were deracemized with isolated yields close to 80%.

13.3 Deracemization of α-Hydroxy Nitriles

Cyanohydrins are versatile and important building blocks in organic synthesis [20]. Consequently, their enantioselective synthesis has attracted considerable attention [21]. In particular, the use of hydroxynitrile lyase has proved to be a general and reliable method for obtaining α-hydroxy nitriles of both configurations [22]. An interesting approach is the lipase- and base-catalyzed synthesis of acyl-cyanohydrin for a synthetic DKR [23]. This is a combination of two reaction systems: the dynamic, base-catalyzed equilibrium between acetone cyanohydrin, acetone, HCN, aldehyde and a racemic cyanohydrin and the lipase-catalyzed enantioselective and irreversible acylation of the hydroxyl group. The combination yields the

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Scheme 13.8 Deracemization of α-hydroxy nitriles.

corresponding stable cyanohydrin ester. This methodology was first published in 1991 and has been quoted frequently [23]. However, the reaction has long reaction times and it suffers from several experimental details which lower the reaction yields. Recently improvements in the original procedures allowed (S)-mandelonitrile acetate to be obtained from the corresponding rac-alcohol in a 97% yield and 98% e.e. while also shortening the reaction time. Best results were obtained with (lipase from C. antarctica type B) (CALB) adsorbed over celite [24] (Scheme 13.8). A similar reaction procedure has also been successfully applied to the deracemization of cyanohydrines containing heterocyclic structures [25].

13.4 Deracemization of α-Amino Acids

For several reasons α-amino acids are ideal substrates for deracemization methods. They racemize easily by base catalysis under a number of conditions and they are racemized in Nature by the intervention of specific amino acid racemases. They are also recognized as substrates by oxidative enzymes to give the corresponding oxoacids, in turn substrates for amino transferases and amino acid dehydrogenases. Several industrial preparations of L- and D-amino acids are based on processes of deracemization [26] or of separate two-steps resolution–racemization [27]. 13.4.1 Deracemization of α-Amino Acids by Stereoinversion 13.4.1.1 Deracemization by Stereoinversion via the Two-enzyme System D-Amino Acid Oxidase and L-Amino Transferase D-Amino acid oxidase is a highly stereoselective flavoenzyme the properties of which are described in detail in Section 13.5.1. The enzymatic production of


α-imino acids or α-keto acids from the corresponding amino acids is a convenient way of producing these compounds. The α-keto acids can be transformed into amino acids in a reaction catalyzed by an amino transferase in the presence of a stoichiometric amount of a second amino acid acting as an amino donor. The αimino acids instead can give an amino acid by means of an amino acid dehydrogenase. Both reactions can be considered overall as equivalent to reductive aminations. A combination of D-amino acid oxidase and L-amino transferase is an example of a deracemization by stereoinversion. The product is an L-amino acid. The reaction catalyzed by amino transferase has an equilibrium constant close to unity, a very unpractical situation leading to uncomplete transformation and to the production of almost inseparable mixtures of amino acids (at least two, the amino acid product and the amino acid used as an amino donor). For preparative purposes it is therefore mandatory to shift the equilibrium to the product side. A recent example of a deracemization procedure based on this coupled enzymatic system is the preparation of L-2-naphthyl-alanine 6 as illustrated in Scheme 13.9 [28]. The reaction occurs in one pot with initial oxidation of the D-amino acid catalyzed by D-amino acid oxidase from Rhodotorula gracilis. The hydrogen peroxide that is formed in stoichiometric amounts is decomposed by catalase. The α-keto acid is the substrate for L-aspartate amino transferase (L-Asp amino transferase), which is able to use L-cysteine sulfinic acid 7 as an amino donor. The reaction is driven towards the products by spontaneous decomposition of the sulfinyl-pyruvate to pyruvate and sulfur dioxide. The deracemization is complete yielding L-2-naphthyl-alanine in a 95% yield and an e.e. of 99.5%. The preparative significance of the transformation is reduced by the low space–time yield due to the extremely low water solubility of the naphthyl amino acid. However, a solid-to-solid bioconversion is possible in principle [28]. Previous examples of a similar combination of enzymes have been published by Fotheringham et al. [29].

Scheme 13.9 Deracemization by stereoinversion using three different enzymes in an equilibrium-shifted transamination.

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Scheme 13.10 Deracemization of D,L-methionine by stereoinversion.

13.4.1.2 Deracemization by Stereoinversion via the Two-enzyme System D-Amino Acid Oxidase and L-Leucine Dehydrogenase In a related approach, D,L-methionine can be efficiently deracemized to obtain the L-enantiomer using a multienzyme system consisting of D-amino acid oxidase, catalase, leucine dehydrogenase, and formate dehydrogenase. The α-keto acid 8 produced from the oxidation of the D-form is transformed into L-methionine 9 in the presence of ammonia, leucine dehydrogenase, and a stoichiometric amount of NADH. The NAD+ thus formed is recycled to NADH with ammonium formate and formate dehydrogenase [30] (Scheme 13.10). The coupling of these two enzymatic systems could find many more applications due to the availability of amino acid dehydrogenases of broader specificity [31]. A series of amino acid dehydrogenases with D-specificity for the preparation of D-amino acids has been applied to the reductive amination of α-keto acids [32]. However, the deracemization of rac-amino acids exploiting this type of enzyme requires an amino acid oxidase with L-specificity, which is a rare enzymatic activity. As an alternative the α-oxo acid, usually available through difficult synthetic procedures, can be used directly. 13.4.1.3 Deracemization by Stereoinversion via the Three-enzyme System L-Amino Acid Oxidase, D-Amino Transferase and Amino Acid Racemase The preparation of D-amino acids with the above three-enzyme system requires enzymes with opposite stereochemical selectivity and a suitable amino acid as a donor. While D-amino acid oxidase is an enzyme, the function of which in Nature is mainly related to the elimination of D-amino acids, L-amino acid oxidases are usually found in aggressive animals (snakes). Bacterial L-amino acid oxidases often show a specific activity that is too low for preparative purposes [33]. Moreover, Damino transferases are less common than the L-specific ones and require more expensive D-amino acids as amino donors.


Scheme 13.11 Deracemization by stereoinversion finalized to the preparation of D-amino acids.

Several inventive procedures have been used for overcoming these difficulties, offering interesting multistep enzyme-catalyzed reactions for deracemization of amino acids. The requirement of an amino donor of D-configuration can be solved by its in situ generation from the L-form by using an amino acid racemase. The procedure reported in Scheme 13.11 describes deracemization of an amino acid involving oxidation with an L-specific enzyme and transamination with a Damino transferase using D-aspartate 10, which is generated from L-aspartate 11 by aspartate racemase, as the amino donor. The oxidative enzyme is defined as an Lamino acid deaminase, a flavoprotein from Proteus myxofaciens [34]. The transamination reaction is shifted towards the product since the oxalacetate 12 formed decarboxylates spontaneously to give pyruvate and carbon dioxide. 13.4.2 Deracemization of α-Amino Acids via DKR

The requisites for successful DKR are that the starting material is racemizable under the reaction conditions while the product is configurationally stable under the same conditions. Due to the industrial importance of amino acids, a number of studies have been devoted to the problem of effective racemization of the unused enantiomer that remained after a standard kinetic resolution [35]. However, racemization methods based on high temperature or extreme pH values are unsuitable for in situ racemization. Thus, if enzymes are involved, the racemization reaction must be carried out under mild conditions, using the catalytic action of a second enzyme (racemase) or a base, exploiting the difference in the pKa value of the α-methine carbon due to difference in the structure between the substrate and product. 13.4.2.1 Deracemization of α-Amino Acids via Enzyme-catalyzed DKR Coupled with In Situ Racemization Several effects can contribute to lowering the pKa of the proton on the αcarbon of an amino acid derivative, as the requisite for racemization. Both the

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transformation of the amine into a NH–CO bond either in a cyclic structure or in an open chain and the presence of the carboxylate in the form of an ester, amide, or thio ester have the effect of an accelerated racemization of the amino acid derivative. The presence of a C=C bond in the β−γ-position also has a strong effect. In order to obtain an effective racemization under the conditions of pH and temperature at which the enzyme is most active, a combination of more than one structural modification is often required. Substrates for which a general method of an enzyme-catalyzed kinetic resolution coupled with in situ racemization is successful are 5-monosubstituted hydantoins 13 (containing both activating NH– CO and CO–NH increments), 4-substituted-2-phenyloxazolin-5-one 14 (in which the amino acid nitrogen is in the form of an imino group and the carboxylate is present as a lactone bond) and N-Boc-amino acid thioesters 15 (in which the amino group is in the NH–CO form, the carboxylate is in the form of a thioester and the R-group is aromatic). The enzymatic kinetic resolution of amino acid esters coupled with an aldehyde-mediated in situ racemization and the enzymatic kinetic resolution of N-acyl amino acids 16 coupled with N-acyl-amino acid racemaseassisted racemization complete the group of potentially effective deracemizations by DKR for amino acids (Scheme 13.12). The Hydantoinase–Carbamoylase System for D-Amino Acid Synthesis 5Monosubstituted hydantoins are α-amino acids cyclically protected at both the carboxyl- and the α-amino group. They can be easily prepared from an aldehyde and isocyanate or by the Bucherer–Bergs synthesis and similar methods. Indeed, the hydantoin synthesis is also a practical method for the preparation of the racemic amino acid. Enzymes belonging to the dihydro-pyrimidinase family hydrolyze hydantoins to the carbamoyl amino acid. The latter can be hydrolyzed in turn to the amino acid by a second enzyme, a carbamoylase. Both enzymes can discriminate between enantiomers and, if their action is cooperative, either the L- or the D-amino acid can be obtained (Scheme 13.10) [36]. What makes the system of special interest is that the proton in the 5-position of the hydantoin ring (it will become the α-hydrogen in the α-amino acid) is considerably more acidic than conventional protons in amino acid esters or amides and much more acidic than the amino acid itself. Thus, the hydantoin can be often racemized in situ at slightly basic pH where the enzymes are still stable and active. If these conditions are met,

Scheme 13.12 Substrates for enzymatic DKR described in Section 13.4.2.


Scheme 13.13 Hydantoinase–Carbamoylase system for the deracemization of α-amino acids via DKR with in situ racemization.

the amino acid can be obtained in one single enantiomeric form in a 100% yield. A bacterial strain was developed at the industrial level and is now applied for the large-scale production of D-p-hydroxy-phenylglycine and other D-amino acids. The use of the whole cell system is necessary due to the limited stability of the carbamoyl-hydrolyzing enzyme and to the different requirements for optimal reactivity. Best carbamoylase activity is obtained by lowering the reaction pH slightly, which is regulated from the development of carbon dioxide from the first reaction step. The D-hydantoinase–D-carbamoylase process has proved successful for the preparation of a large number of D-amino acids [37] (Scheme 13.13). Hydantoinase–Carbamoylase System for L-Amino Acid Synthesis Despite a number of reports of strains with L-selective hydantoin-hydrolyzing enzymes [38] the commercial application of the hydantoinase process is still restricted to the production of D-amino acids. Processes for the production of L-amino acids are limited by low space–time yields and high biocatalyst costs. Recently, a new generation of an L-hydantoinase process was developed based on a tailor-made recombinant whole cell biocatalyst. Further reduction of biocatalyst cost by use of recombinant Escherichia coli cells overexpressing hydantoinase, carbamoylase, and hydantoin racemase from Arthrobacter sp. DSM 9771 were achieved. To improve the hydantoin-converting pathway, the level of expression of the different genes was balanced on the basis of their specific activities. The system has been applied to the preparation of L-methionine: the space–time yield is however still limited [39]. Improvements in the deracemization process from rac-5-substituted hydantoins to L-amino acids still requires a more selective L-hydantoinase.

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Scheme 13.14 4-Substituted-2-phenyloxazolin-5-one as substrates for lipases or esterases, with opposite selectivity allowing DKR with in situ racemization.

Enzymatic Kinetic Resolution of Phenyl Oxazolinones Coupled with In Situ Basecatalyzed Racemization Phenyl oxazolinones are a protected form of amino acids that can be obtained by cyclization of N-acyl-amino acids. Their structure is similar to hydantoins with the relevant difference that the carboxylate is in a lactone-type (–CO–O–) form while in hydantoins it is in a lactame type form (–CO–N–). Lipases are known to be not well suited for amide bond transformation. Therefore oxazolinones were designed as alternative structures for a DKR similar to the one described for hydantoins but involving lipases as resolving agents. Rapid enolization under slightly basic conditions should ensure the possibility for DKR. The oxazolinone 17 derived from phenylalanine is an excellent substrate for porcine pancreatic lipase and lipase from Aspergillus niger, although the transformation required rather long reaction times (20 h) (Scheme 13.14). The interest in this specific method is that the two enzymes show opposite enantiopreference with this substrate. Altogether the hydrolysis has useful characteristics only for the preparation of phenylalanine [40]. Other oxazolinones are transformed with a much lower e.e. and longer reaction times. An alternative approach to the DKR of phenyl oxazolinones is represented by alcoholysis in organic solvents. The lipase from Pseudomonas cepacia was used for this purpose. In organic solvent with low water activity non-enzymatic hydrolysis proceeds very slowly but the rate of enolization of the C-4 proton is sufficiently rapid so that 100% of the substrate is converted into product. By using methanol as the nucleophile, methanolysis of oxazolones proceeded at a useful rate to furnish methyl esters of N-benzoyl-L-α-amino acids. The optical purity of the products ranged from 66 to 98% e.e. [41]. A method based on the DKR of 4-substituted-2-phenyloxazolin-5-one has been applied to the large-scale synthesis of N-benzoyl-L-t-leucine-butyl ester 19, an important non-natural amino acid intermediate of several biomimetic peptides. The procedure is based on the alcoholysis of 4-t-butyl-2-phenyloxazolin-5-one 18 with n-butanol catalyzed by the lipase from Mucor miehei (Scheme 13.15) [42]. The unreactive enantiomer is racemized in situ by a base-catalyzed reaction. The


Scheme 13.15 4-t-Butyl-2-phenyloxazolin-5-one as substrates of lipase from M. miehei for the DKR with in situ racemization.

Scheme 13.16 Enzymatic kinetic resolution of N-Boc-amino acid thioesters coupled with base-catalyzed racemization.

reaction can be performed at a substrate concentration of 200 g/l in 24 h. Yields are 90% and the e.e. is >95%. Enzymatic Kinetic Resolution of N-Boc-Amino Acid-Thioesters Coupled with Basecatalyzed Racemization Recently, a new method leading to the preparation of a number of aryl-glycines of the L-configuration has been published. The method is based on the hydrolysis of N-Boc-amino acid thioesters 15 catalyzed by an industrial preparation of the protease subtilisin (Scheme 13.16) [43]. In thioesters, the acidity of the α-hydrogen is higher as compared to to the corresponding oxo-esters, amides, or acids. The enzymatic transformation of a thioester into the corresponding carboxylate with a higher pKa of the α-proton makes up the basis for DKR, provided that the enzymatic systems withstand the basic conditions required for substrate racemization. This concept has been applied in the DKR of α-alkyl thioesters. For application in amino acid chemistry racemic compounds were designed for this novel application. Moreover, subtilisin Carlsberg showed a good activity on these N-Boc amino acid derivatives with reaction rates comparable to the one observed with the regular esters. The reaction was performed in a water–methyl tertiary-butyl ether (MTBE) two-phase system in the presence of subtilisin. The reaction reached 50% conversion after 24 h: at

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this point the mixture was depleted of the L-enantiomer, which is the substrate of the enzyme. When trioctyl-amine was added as a base to the reaction mixture, such racemizing conditions generated the L-enantiomer and the enzymatic hydrolysis started again reaching complete conversion after 24 h. The N-Boc-L-phenylglycine was isolated in a 95% yield and >99% e.e. The structural requirements are an aryl or vinyl group on the α-carbon, a variety of alkyl groups on the sulfur atom, and a carbonyl group on the nitrogen. In addition, in this case an application to the preparation of the D-enantiomers would require catalysis by an enzyme with opposite enantioselectivity. The Enzymatic Kinetic Resolution of Amino Acid Esters Coupled with AldehydeMediated In Situ Racemization The reaction of the amino group of amino acid derivatives with aromatic aldehydes can lead to racemization through the reversible formation of imines. DKR can be obtained by combining the racemization with an enzyme-catalyzed preferential hydrolysis of one enantiomer. This principle has been applied to the DKR of a series of amino acid esters using pyridoxal phosphate or salicylic aldehyde. As an example, the esters were treated with an industrial preparation of subtilisin in the presence of 20 mol% of pyridoxal in a 19 : 1 mixture of 2-methyl-2-propanol and water. During the reaction course the hydrolyzed amino acid is insoluble and precipitates in the reaction medium. The remaining ester is racemized by imine formation with pyridoxal phosphate. The rate of racemization of the amino acid is only marginal due to the higher pKa of the α-proton and the low solubility. Yields exceeding 90% of the L-enantiomer are obtained with an e.e. ranging from 90 to 98%. With this method Phe, Tyr, Leu, NorVal, and NorLeu of the L-configuration have been obtained. The drawback of the method is the requirement of a sensible amount of the expensive racemization catalyst (Scheme 13.17) [44]. A similar concept was applied to the DKR of phenyl glycine esters catalyzed by lipases in an aminolysis reaction [45].

Schem 13.17 Kinetic resolution of amino acid esters coupled with pyridoxalphosphate mediated in situ racemization.


Enzymatic Kinetic Resolution of N-Acyl Amino Acids Coupled with Racemization by N-Acyl Amino Acid Racemase Acylases are enzymes hydrolysing the N-acetyl derivatives of amino acids. They require the free carboxylate for activity and have long been used for the kinetic resolution of amino acids. The unreacted enantiomer is usually racemized in a separate step by treatment with acetic anhydride. While acylases from hog kidney have an L-specificity, bacterial acylases with L- and D-specificity of various origins have been isolated and used for the kinetic resolution of N-acetyl amino acids. An industrial process for the production of L-Met and other proteinogenic and non-proteinogenic L-amino acids such as L-Val, L-Phe, L-Norval, or L-aminobutyric acid has been established. Currently, several hundred tons per year of L-methionine are produced by this enzymatic conversion using an enzyme membrane reactor [46]. The starting material for the acylase process is a racemic mixture of N-acetylamino acids 20 which are chemically synthesized by acetylation of D,L-amino acids with acetyl chloride or acetic anhydride in alkali via the Schotten–Baumann reaction. The kinetic resolution of N-acetyl-D,L-amino acids is achieved by a specific L-acylase from Aspergillus oryzae, which only hydrolyzes the L-enantiomer and produces a mixture of the corresponding L-amino acid, acetate, and N-acetyl-Damino acid. After separation of the L-amino acid by a crystallization step, the remaining N-acetyl-D-amino acid is recycled by thermal racemization under drastic conditions (Scheme 13.18) [47]. In a similar process racemic amino acid amides are resolved with an L-specific amidase and the remaining enantiomer is racemized separately. Although the final yields of the L-form are beyond 50% of the starting material in these multistep processes, the efficiency of the whole transformation is much lower than a DKR process with in situ racemization. On the other hand, the structural requirements for the free carboxylate do not allow the identification of derivatives racemizable in situ: therefore, the racemization requires

Scheme 13.18 The N-acylase-catalyzed kinetic resolution of N-acyl amino acids coupled with racemase induced racemization.

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the application of specific enzymes. A whole cell system has been developed expressing the genes encoding for N-acyl amino acid racemase and L-aminoacylase from Deinococcus radiodurans in E. coli. This system was used for the deracemization of D,L-N-Ac-homophenylalanine. The L-amino acid was obtained in quantitative yield when the expression of the two enzymes was regulated properly. The cells could be used several times in the biotranformation. Amino Acid Oxidase and Chemical In Situ Reduction of the Initially Formed Imino Compound A simple and interesting procedure for the deracemization of αamino acids was introduced by Soda [48], who combined the oxidation of the Denantiomer of D,L-proline to dehydro-proline with a chemical reduction of the imine 21 in on -pot, thereby restoring the racemic mixture. If the reaction in the first step is completely enantioselective, the e.e. of the amino acid after one cycle is 50%. Repeating the reaction in successive cycles raises the e.e. close to 100% (Scheme 13.19). In a similar way, D,L-pipecolic acid was converted to L-pipecolic acid [49]. However, the use of sodium borohydride, which slowly decomposes in aqueous solution, causes a significant rise in the pH value, which has to be controlled and which interferes with the necessary repeated cycles. Turner and Fotheringham expanded the applicability of the method, using milder and water stable reducing agents [50] and applying amino acid oxidases with either D- or L-specificity. Thus, using L-amino acid oxidase from P. myxofaciens and various amine–borane complexes or D-amino acid oxidase from porcine kidney and sodium cyanoborohydride, the preparation of several natural and non-natural enantiopure D- and Lamino acids was achieved, respectively [51]. In a more recent report, several β- and γ-substituted α-amino acids were deracemized using D-amino acid oxidase from Trigonopsis variabilis and sodium cyanoborohydride or sodium borohydride [52] (Scheme 13.20). Aiming at the development of more efficient whole cell biocatalysts, deracemization of 4-chlorophenylalanine was reported using E. coli cells expressing an

Scheme 13.19 Chemoenzymatic deracemization with D-amino acid oxidase and chemical reduction.

13.5 Useful Enzymes for Deracemization Methods

Scheme 13.20 Chemoenzymatic deracemization with L-amino acid oxidase and chemical reduction. L-amino acid amino transferase from Sinorhizobium meliloti ATCC 51124. The enantiopure L-α-amino acid was obtained in high optical yield via the tandem action of D-amino acid dehydrogenase from the E. coli host cell (induced by L-Ala in the medium) and the cloned L-amino acid amino transferase from S. meliloti [53].

13.5 Useful Enzymes for Deracemization Methods 13.5.1 Amino Acid Oxidases

The enzymes discussed in this paragraph are D- and L-amino acid oxidases, the presence of which has been detected in numerous organisms and tissues. They were only recently identified in bacteria and the corresponding enzymes were isolated (for a review see [54]). These enzymes are flavin adenine dinucleotide (FAD)-containing flavoproteins that catalyze the oxidative deamination of amino acids (Scheme 13.21). The flavin co-factor is reduced with concomitant oxidation of the amino acid into the corresponding imino acid, which spontaneously hydrolyzes to the α-keto acid and ammonia: the flavin co-factor is then reoxidized by molecular oxygen with the production of hydrogen peroxide. Both D- and L-amino acid oxidases deaminate a variety of amino acids, particularly those having a hydrophobic side chain, but are practically inactive on acidic amino acids (which are deaminated by specific glutamate and aspartate oxidases) and are strictly stereospecific. 13.5.1.1 D-Amino Acid Oxidase (EC 1.4.3.3) The most investigated D-amino acid oxidases are those from the yeasts R. gracilis and T. variabilis and the one from pig kidney. In solution D-amino acid oxidases are homodimers, with each monomer constituted by ∼350–370 residues and containing one molecule of non-covalently bound FAD co-factor. Mammalian and

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Scheme 13.21 Reaction catalyzed by D-amino acid oxidase and L-amino acid oxidase.

fungal D-amino acid oxidases differ significantly in their affinity for the co-enzyme FAD, which is most tightly bound to those from microorganisms and in kinetic efficiency (the Vmax values on D-alanine as a substrate at air saturation and 25 °C are 110, 60 and 9.5 units/mg protein for R. gracilis, T. variabilis and pig kidney Damino acid oxidases, respectively). Yeast D-amino acid oxidases show a high stability in the 6.0–8.2 pH range and a decrease in stability at higher pH values, while pig kidney D-amino acid oxidase shows a maximum stability at pH 9.5 and a decrease at acidic pH values. Rhodotorula gracilis D-amino acid oxidase is stable up to 35 °C, T. variabilis D-amino acid oxidase is stable up to 40 °C (it is fully inactivated at 65 °C), and the stability of pig kidney D-amino acid oxidase is greater than that of R. gracilis D-amino acid oxidase. D-Amino acid oxidases from different sources show different substrate specificity. Known microbial D-amino acid oxidases were recently classified into two groups [55]: D-amino acid oxidases from microorganisms such as Fusarium oxysporum, C. parapsilosis, and Candida boidinii are highly specific for D-Ala, while R. gracilis and T. variabilis D-amino acid oxidases show maximum activity with D-Met, D-Trp, D-Val, and D-Phe. The kinetic mechanism of the reaction catalyzed by D-amino acid oxidase on D-Ala is consistent with a ternary complex (sequential) mechanism: the rate-limiting step is the rate constant for product release from the reoxidized enzyme for pig kidney D-amino acid oxidase and the rate constant for flavin reduction for R. gracilis D-amino acid oxidase. According to structural and sequence similarities of the FAD-binding region, D-amino acid oxidase is classified as a member of the glutathione reductase family (subgroup GR2). Each D-amino acid oxidase monomer can be divided into a FADbinding (basal) domain and a dimerization (apical) domain [56]. The basal domain contains the typical nucleotide-binding domain, while the apical domain is formed by a pseudo-β-barrel structure consisting of two twisted β-sheets: residues belonging to this region form the major part of the active site and the whole dimer interface. Structural and mechanistic studies on D-amino acid oxidases from various sources suggest that the enzymatic reaction involves a direct hydride transfer from the substrate D-amino acid to the oxidized flavin. The most striking feature of D-amino acid oxidases is that no active site residues are directly involved in the chemical steps of catalysis (Figure 13.1a). The D-amino acid side chain makes contacts with the wide cavity representing the upper part of the active site cleft, which is mostly hydrophobic: this provides a rationale as to why R. gracilis D-amino acid oxidase prefer D-amino acids carrying an aromatic and/or hydrophobic side chain to polar ones and for its capacity to accommodate large compounds,


Figure 13.1 Schematic representation of interactions at the active site of (a) R. gracilis D-amino acid oxidase in complex with CF3-D-alanine (pdb 1COL), (b) C. rhodostoma L-amino acid oxidase in complex with citrate (pdb IF8R), and (c) B. stearothermophilus alanine racemase (pdb 1SFT).

as in the case of the antibiotic cephalosporin C. It also explains the reluctance of R. gracilis D-amino acid oxidase to act on amino acids carrying a polar and, in particular, a charged side chain. Protein engineering studies have allowed modulation of D-amino acid oxidases’ oligomerization state, stability (which is significantly increased in the immobilized form), FAD binding, and substrate specificity (for a review see [57]). The residue

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Met213 of R. gracilis D-amino acid oxidase has been proposed as the most important residue for determining the substrate specificity of the enzyme. In fact, the M213R mutant evolved the ability to oxidize acidic D-amino acids such as D-Asp. Recently, R. gracilis D-amino acid oxidase was employed in the resolution of racemic mixtures of D,L-naphthyl amino acids and the M213G mutant showed a higher catalytic efficiency on D-naphthyl alanine and D-naphthyl glycine [28]. Subsequently, five mutants displaying an altered substrate specificity were selected by a directed evolution approach of R. gracilis D-amino acid oxidase: these Damino acid oxidase variants have a better catalytic efficiency for all the substrates used [57]. 13.5.1.2 L-Amino Acid Oxidase (EC 1.4.3.2) L-Amino acid oxidase is a flavoenzyme that catalyzes the oxidative deamination of L-amino acids. L-Amino acid oxidase activities have been detected in mammals, birds, reptiles, invertebrates, molds, and bacteria [54]. L-Amino acid oxidases show the typical absorption spectrum due to the presence of a molecule of noncovalently bound FAD per subunit (with maxima at 465 and 380 nm): they behave like flavoprotein oxidases, as in the case of D-amino acid oxidase. L-Amino acid oxidase isolated from rat liver was reported to utilize flavin mononucleotide (FMN) as a co-enzyme, but since it is more active on L-hydroxy acids than on amino acids, it was thus considered as an L-hydroxy acid oxidase. Even a partially purified Lamino acid oxidase from turkey liver appeared to have FMN as a co-factor. Snake venom L-amino acid oxidases are the best-studied members of this protein family but their function is still debated: they play a role in inducing apoptosis and affecting platelets and are postulated to be toxins. L-Amino acid oxidases from bacteria, fungi, and plants are involved in the utilization of nitrogen sources. Snake venom L-amino acid oxidases are usually homodimeric flavoglycoproteins with a molecular mass of ∼110–150 kDa: the scattering in molecular mass is partially due to the different sugar content. Snake venom L-amino acid oxidases also show different isoelectric points (from 4.4 to 8.1) and frequently more than one type of the enzyme is produced. Snake venom L-amino acid oxidases share sequence similarity around the FAD-binding site with human monoamine oxidase and microbial L-amino acid oxidases and with mouse interleukin 4-induced Fig1-protein, but no similarity is observed with D-amino acid oxidases. The best substrates for ophidian L-amino acid oxidases are aromatic or, most generally, hydrophobic amino acids, polar and basic amino acids being deaminated at much lower rates: Glu, Asp, and Pro are not oxidized by L-amino acid oxidase. L-Amino acid oxidase is also active on ring-substituted aromatic amino acids, as well as on seleno cysteinyl derivatives. The substrate specificity depends on the source of the enzyme (e.g. Ophiophagus hannah L-amino acid oxidase also oxidizes Lys and Orn) and on the pH. The Vmax of the reaction of Crotalus adamanteus L-amino acid oxidase at pH 8.5 is ∼12 and ∼6 units/mg protein on L-Arg and L-Leu, respectively, while a higher activity is observed for the L-amino acid oxidase from O. hannah on L-His (∼60 units/mg protein). The kinetic mechanism of L-amino acid oxidase from C. adamanteus follows that in Scheme 13.13 and is substantially similar to that reported for D-amino acid oxidase [54]. At high pH


and low oxygen concentration (5 × 10−3 M) a sequential mechanism is effective. The catalytic mechanism of flavin reduction in the L-amino acid oxidase reaction was proposed to be similar to that of D-amino acid oxidase. L-Amino acid oxidase undergoes two types of reversible inactivation: one obtained by raising the pH from 5.5 to 7.5 and the temperature from 25 to 38 °C, while the second one is caused by storage at –5 to –60 °C and depends on the pH (it is favored by acidic pH values) and on the ionic composition of the storage buffer. In both cases reactivation is achieved by incubating the enzyme at pH 5 and 38 °C for 1 h. Snake venom L-amino acid oxidases should be maintained in the dark at 4 °C and near neutral pH to avoid inactivation. The X-ray crystallographic structure of Calloselasma rhodostoma L-amino acid oxidase indicates that it is functionally a dimer in which each subunit is constituted by three domains: an FAD-binding domain, a substrate-binding domain, and a helical domain [58]. A funnel is formed at the interface between the latter domains and provides substrate access to the active site. Thus, the mode of substrate binding (Figure 13.1b) and access significantly differs between L-amino acid oxidase and D-amino acid oxidase. Recently, L-amino acid deaminase (EC 1.4.3.x) activities have been identified, particularly from the Proteus genus [59]. This enzyme, constituted by ∼370 residues, is an FAD-containing L-amino acid oxidase flavoprotein that uses molecular oxygen to convert L-amino acids into the corresponding α-keto acids and ammonia but does not produce hydrogen peroxide. L-amino acid deaminase prefers amino acids with aliphatic, aromatic, and sulfur-containing side chains (the best substrates are L-Leu, L-Phe, L-Met, and L-Trp) and its kinetic efficiency is quite low because of the low Vmax value (≤2 units/mg protein). 13.5.2 Amino Acid Racemases

A variety of amino acid racemases have been identified in bacteria, archaea, and eukaryotes. They are classified into two groups: pyridoxal 5′-phosphate (PLP) -dependent and -independent enzymes. Therefore, racemization can be achieved via two mechanisms: through a chirally unstable Schiff base intermediate with an aromatic aldehyde serving as co-factor PLP (Scheme 13.22a) and by a two-base mechanism without co-factor (Scheme 13.22b). 13.5.2.1

PLP-dependent Racemases

Alanine Racemase (EC 5.1.1.1) Alanine racemase is a PLP-dependent bacterial enzyme that catalyzes the racemization of L- and D-alanine exclusively (for a review see [60]). The enzyme from Salmonella typhimurium is an exception since it also accepts L-Ser, L-homo-Ser, and L-Cys as substrates. Alanine racemase, the most studied member of the PLP-dependent racemases, plays a major role in the bacterial growth by providing D-Ala for the peptidoglycan assembly and cross-linking.

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Scheme 13.22 Amino acid racemization by (a) PLP-dependent alanine racemase and (b) PLP-independent α-amino acid racemase.

Alanine racemase was purified and cloned form various sources. Interestingly, two distinct genes were identified in a number of genome sequences, for example, E. coli, Bacillus subtilis, Pseudomonas aeruginosa, and so on. Site-directed mutagenesis experiments and X-ray crystallography of alanine racemase from Bacillus stearothermophilus (Figure 13.1c) [61] suggested that the


enzyme uses two catalytic bases, namely Tyr265 and PLP-binding Lys39. Lys39 abstracts the α-H from D-Ala and adds the α-H to the intermediate to form D-Ala: Tyr265 is instead the corresponding catalytic residue acting on L-Ala. This mechanism requires that the α-H abstracted from the substrate is transferred between the side chain of Lys39 and that of Tyr265, otherwise the enzyme reaction would stop after a single turnover. A second mechanism was also proposed, in which the carboxyl group of the substrate participates in catalysis mediating the α-H transfer between the two catalytic residues. Furthermore, alanine racemase is a bifunctional enzyme, that is, besides racemization, it also catalyzes transamination at low pH (∼6). Alanine racemases are homodimeric enzymes of an apparent molecular mass of ∼76 kDa, containing two molecules of PLP as co-enzyme [61]: the known enzymes from different sources are highly homologous. With regard to the kinetic properties, the Km values for D- and L-Ala determined at 30 °C for the Pseudomonas fluorescens alanine racemase are ∼12 and 19 mM, respectively and the Vmax values for racemization are ∼1200 and 2200 units/mg protein, respectively. The thermophilic alanine racemase from B. stearothermophilus is quite stable to heat treatment (up to 75 °C for 1 h) while the mesophilic one from B. subtilis is stable up to 55 °C under the same conditions. Serine Racemase (EC 5.1.1.16) Serine racemases have been discovered in both bacteria and eukaryotes (for a review see [60, 62]. In the latter organisms, serine racemase catalyzing the conversion of L-Ser to D-Ser was at first discovered in the silkworm Bombyx mori: it is a PLP-dependent racemase which is also active on LAla (∼6% of the activity on L-Ser). A serine racemase was also purified from rat brain (and a serine racemase cDNA was cloned from mouse brain). Mammalian serine racemase shows sequence similarity with L-threonine dehydratase from various sources: all the active site residues of the latter enzyme are also conserved in mouse serine racemase. Mammalian serine racemase is a member of the fold-type II group of PLP enzymes (similarly to L-threonine dehydratase, D-serine dehydratase, and so on) and distinct from alanine racemase, which belongs to the fold-type III group. Mouse serine racemase shows a low kinetic efficiency: the Km values for L- and D-Ser are ∼10 and 60 mM, respectively and the Vmax values with L- and D-Ser are 0.08 and 0.37 units/mg protein (less than 0.1% of those of alanine racemase on L- and D-Ala, see above). On the other hand, bacterial serine racemase from Enterococcus gallinarum (VanT) is instead homologous to alanine racemase. It is constituted by 698 amino acids with an N-terminal domain containing 10 predicted transmembrane segments and a C-terminal domain showing 31% of sequence identity with alanine racemase from B. stearothermophylus. The active site residues of alanine racemase are also conserved in VanT: this observation indicates that the reaction catalyzed by VanT probably proceeds via a two-base mechanism analogously to that proposed for alanine racemase (see above). Interestingly, its physiological role is related to vancomycin resistance.

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13.5.2.2 PLP-independent Racemases The members of this class follow a two-base mechanism involving two cysteine residues as the conjugated catalytic acid and base for the abstraction of the α-H from both amino acids. Glutamate Racemase (EC 5.1.1.14) Glutamate racemase genes have been cloned from a number of bacterial sources [60]. Glutamate racemases are monomeric enzymes of 28–30 kDa molecular mass. Glutamate racemases from Pediococcus pentosaceus is absolutely specific for glutamate and the one from E. coli is instead activated (∼100-fold) by UDP-N-acetyl-muramoyl-L-alanine, a precursor of the peptidoglycan layer of the cell walls. A region of 21 amino acids at the N-terminus of the E. coli enzyme is responsible for the activation. Glutamate racemase contains two active site cysteines per enzyme molecule and one of these is essential for catalysis: a thiolate from one cysteine abstracts the α-H of the substrate and the other cysteine thiol delivers a proton to the opposite face of the resulting carbanionic intermediate. X-ray diffraction studies and site-directed mutagenesis experiments of Aquifex pyrophilus glutamate racemase indicated that Cys73 is responsible for the deprotonation of D-Glu and Cys184 is responsible for the deprotonation of L-Glu. Because of the large separation between the pKa values of the α-H of the amino acid (∼21) and that of a thiol (∼10), an unidentified system is required for facilitating proton abstraction. Asp10 and His186 of Lactobacillus fermenti glutamate racemase have been proposed for assisting the function of Cys73 and Cys184, respectively, although crystallographic data on A. pyrophilus glutamate racemase indicate that the histidine is too far away from the second cysteine for a direct interaction. With regard to the kinetic properties of L. fermenti glutamate racemase, the Vmax value is ∼70 s−1 (∼130 units/mg protein) and the Km is 0.3 mM for both L- and D-Glu [63]. The highest kinetic efficiency is observed at pH 8–8.5. Aspartate Racemase (EC 5.1.1.13) Aspartate racemase produces D-Asp from the corresponding L-isomer for the biosynthesis of the peptidoglycan layer of bacterial cell walls. This enzyme has been identified in various Lactobacillus and Streptococcus strains, as well as in archaea such as Desulfurococcus and Thermococcus strains [60]. Analogously to glutamate racemase, Streptococcus thermophylus aspartate racemase does not require co-factors and contains an essential cysteine, thus suggesting that a two-base mechanism is also active in aspartate racemase. The X-ray diffraction analysis of Pyrococcus horikosii aspartate racemase shows that cysteines 82 and 194 are located on both sides of a cleft between two protein domains where they probably act as the catalytic acid and base. Interestingly, PLP-dependent aspartate racemases have been recently reported, specifically from Thermoplasma acidophilum and Scarpharca broughtonii. Additional Racemase Activities Further useful amino acid racemases are [62]: phenylalanine racemase (EC 5.1.1.11), the only racemase that requires ATP to activate the substrate, proline racemase (EC 5.1.1.4) from Clostridium stricklandii, and amino acid racemase (EC 5.1.1.10) from Pseudomonas putida. It shows a


wide substrate specificity comprising D-Ala, D-Phe, D-Arg, D-Asp, L-nor-Leu, L-Ser, L-Met, and L-Orn, arginine racemase (EC 5.1.1.9) active on L-Lys, L-Arg, L-Orn, and so on, and racemases active on α-amino acid derivatives, such as the one active on α-amino lactams (EC 5.1.1.15) and that active on α-acetyl amino acids (NAAR). 13.5.2.3 Mandelate Racemase (EC 5.1.2.2) Mandelate racemase is a PLP-independent well-characterized racemase (for a review see [62, 64]. Because of its remarkable stability and broad substrate specificity, it has been considered an ideal candidate for deracemization reactions. The gene encoding P. putida mandelate racemase has been cloned and its threedimensional structure has been solved: it is an octameric enzyme constituted by subunits of 39 kDa, with each subunit consisting of a circular arrangement of eight parallel β-sheets. In the active site a catalytically active Mg2+ ion is positioned through the interaction with an array of aspartate, glutamate, and lysine residues. Besides its natural substrate mandelate, it also racemizes the corresponding amide derivatives. Furthermore, the aromatic system of mandelate may be expanded to a naphthyl system and heteroaromatic analogs of mandelate, as well as vinyl glycolate, can also be accepted by the enzyme. The broad substrate tolerance is mainly due to a remarkable plasticity of the hydrophobic binding pocket within the active site. A predictive substrate model for mandelate racemase has been proposed [64]. As stated above for glutamate racemase, the main obstacle during the interconversion of mandelate enantiomers is the abstraction of the α-H (pKa ∼29) for generating the corresponding achiral enolate intermediate. This step is facilitated by the binding of both enantiomers within the active site through a tight network of hydrogen bonds and salt bridges involving the residues Glu317 and Lys164 and Mg2+. 13.5.3 Transaminases

Amino transferases (transaminases) catalyze the PLP-dependent reversible transfer of an amino and keto group between an amino acid donor and a keto acid acceptor yielding new amino acid and keto acid product, according to the equation in Scheme 13.23. When the amino donor has the L-configuration and an L-amino acid transaminase is used, the product amino acid will have an L-configuration and when the amino donor has the D-configuration and a D-amino transaminase is used, the product amino acid will have a D-configuration. The equilibrium constant for the reaction is typically close to unity. L-Amino transaminase activities are ubiquitous in Nature since they are involved in the biosynthesis of most natural amino acids. On the other hand, D-amino transaminase activities have been identified in bacteria, mostly in the Bacillus strains and are involved in the production of D-amino acids for the peptidoglycan layer of the cell wall. The mechanism is well established, with PLP shuttling

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Scheme 13.23 Reaction catalyzed by amino transferase.

between pyridoxal and pyridoxamine forms in the reversible transfer of amino and keto groups between an amino acid and a keto acid substrate pair. The reaction proceeds by ping-pong kinetics. Transaminases possess many features appropriate for efficient biocatalysts, such as high turnover numbers and no requirement for external recycling of the co-factor. Because of the wide substrate tolerance of many amino transferases such as tyrosine amino transferase and branched-chain amino transferases from E. coli, these enzymes have been largely employed in the enantiospecific preparation of non-proteinogenic amino acids. These include straight-chain alkyl, diacid, branched-chain, aromatic, and bifunctional amino acids [65]. 13.5.3.1 L-Amino Transferases (EC 2.6.1.x) The most commonly used L-amino transferase activities for L-amino acid synthesis are the ones from E. coli, which can be used in whole cell or immobilized systems. They include the following. First, aspartate amino transferase (EC 2.6.1.1), the E. coli gene, named aspC, which encodes for a 88 kDa protein that is active as homodimer. Branched-chain amino transferase (EC 2.6.1.42), the enzyme from E. coli, which is a hexamer composed of identical subunits of ∼34 kDa and has a three-dimensional structure that has been solved, Tyrosine amino transferase (EC 2.6.1.5): in E. coli the tyrB gene encodes for the protein involved in the final step of Tyr biosynthesis, that is the transamination of p-OH-phenylpyruvate to tyrosine using L-Glu as the amino donor. L-Ornithine δ-amino transferase (EC 2.6.1.13): it catalyzes the reversible transamination of the δ-amino group from L-ornithine to α-ketoglutarate producing L-glutamate-γ-semialdehyde and L-Glu. This activity has been identified in many bacteria, mainly belonging to the Bacillus strains. The B. subtilis rocD gene encodes for a 44 kDa protein. A number of other amino transferases have been identified: 4-amino-butyrate-2-ketoglutarate transaminase (EC 2.6.1.19), which has been used to prepare the herbicide L-phosphinothricin and D-phenylglycine amino transferase from Pseudomonas stutzeri, which shows a very narrow substrate tolerance and causes the stereoinversion of the transaminated amino acid (it catalyzes the transamination of D-Phe using α-ketoglutarate as amino donor to form L-Glu) [66]. 13.5.3.2 D-Amino Transferases (EC 2.6.1.21) Among the D-amino transferase activities, the one from Bacillus sp. YM1 has been widely investigated: the gene has been cloned, the protein has been purified and

References

its three-dimensional structure has been solved [67]. This enzyme is a dimer of overall molecular mass 65 kDa and each monomer requires one PLP molecule as a co-enzyme. It does not show sequence identity with L-amino transferase. The fold of D-amino transferase is completely different from that of any of the other known enzymes that utilize PLP. However, there are some striking similarities between the active sites of D-amino transferase and the corresponding enzymes that transaminate L-amino acids, such as L-aspartate transaminase. Most recently the D-amino transferases from Staphylococcus haemolyticus, Bacillus licheniformis and Bacillus sphaericus have been studied. Due to their broad substrate specificity, D-amino transferases are useful in the production of D-amino acids, especially when a thermostable enzyme is employed.

13.6 Summary and Outlook

Deracemization methods constitute an efficient access to chiral compounds as single enantiomers starting with a racemic mixture. Entirely enzymatic methods consist of multistep enzyme-catalyzed reactions as in the deracemization of amino acids catalyzed by an amino acid oxidase and an amino transferase (Section 13.4.1). The limitation of this method comes from the availability of the oxidative enzymes with both stereochemical preferences, in the thermodynamic equilibrium conditions in the reaction catalyzed by amino transferases, and in the stereochemical preference of this enzyme. When an enzymatic resolution process is coupled with a racemization step, a specific racemizing enzyme can be used. The advantage is that the process can be performed in one single reaction act, in mild conditions (see for instance Sections 13.2.1 and 13.4.1.3). However, racemases are in general delicate enzymes, with very narrow substrate specificities (Section 13.5.2). Excellent results can be obtained when a hydrolytic enzyme is coupled with a base in racemizing conditions. Besides the industrial system hydantoinase/carbamoylase for the DKR of 5-substituted hydantoins leading to D-amino acids, other methods relying on a base-catalyzed racemization like oxazolone and thioester hydrolysis are promising for large-scale applications. The constraint again is the availability of hydrolytic enzymes with opposite enantiospecificity. These methods compare favorably with well-known procedures based on transition metal catalysis.

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14 Nitrilases from Filamentous Fungi Ludmila Martínková, Vojteˇch Vejvoda, Ondrˇej Kaplan, Vladimír Krˇen, Karel Bezouška and Maria Cantarella

14.1 Introduction

Nitrilases constitute branch 1 of the nitrilase superfamily, which contains enzymes acting on various non-peptide carbon–nitrogen bonds [1, 2]. The occurrence of nitrilases in microorganisms appears to be rather rare, as indicated by the search for nitrilase genes in sequenced microbial genomes [3]. Nevertheless, culture enrichment techniques have allowed microorganisms with nitrilase activities to be isolated. In addition, uncultured environmental samples were explored by the metagenomic approach, providing over 200 unique nitrilase sequences, out of which 137 were examined for substrate specificity and enantioselectivity [4]. The well-characterized nitrilases listed in recent reviews [5–9] have mostly been of bacterial origin. Few eukaryotic nitrilases have been purified and characterized to date. Among these, the enzymes best characterized at the protein and gene level belong to plant nitrilases, that is primarily AtNIT1-4 enzymes from Arabidopsis thaliana (for reviews see [2, 6]). Knowledge of fungal nitrilases was very limited until recently. Databases contain about 40 sequences of fungal nitrilases or cyanide hydratases, which are largely putative proteins disclosed by genome sequencing. Two nitrilases from the genus Fusarium were purified and characterized but their amino acid sequences have not been reported [10, 11]. Very recently, our work improved knowledge of fungal nitrilases, in particular those from Aspergillus niger and Fusarium solani. On the basis of sequence data and biochemical characterization, this chapter will compare the structure and enzymatic properties of nitrilases from filamentous fungi with those of prokaryotic and plant nitrilases as well as other related enzymes like cyanide hydratases and cyanide dihydratases. The biotechnological potential of fungal nitrilases will be evaluated.


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14.2 Distribution and Evolutionary Relationship of Fungal Nitrilases 14.2.1 Molecular Genetic Analysis

The enzymes from A. niger K10 [12] and F. solani O1 [13] were the first biochemically characterized fungal nitrilases, the partial amino acid sequences of which have been reported [14, 15]. Within the time elapsed since the purification and characterization of the nitrilase from A. niger K10 [14], the corresponding gene has been sequenced and translated to the amino acid sequence (see Figure 14.1). The search for related proteins using the protein blast program (http://www.ncbi.nlm. nih.gov/BLAST/) revealed high similarity (≥ 85% identities and ≥ 92% positives) of the purified nitrilase from A. niger to putative cyanide hydratases/nitrilases encoded in A. niger, Neosartorya fischeri, Aspergillus fumigatus, Aspergillus terreus, Aspergillus clavatus and Aspergillus nidulans. Out of these proteins, the putative nitrilase from A. niger CBS 513.88 (XP_001389844) showed the highest similarity with the purified protein (98.8%). N-Terminal sequencing and mass spectrometry analysis of the purified nitrilase from F. solani O1 revealed four peptide fragments that were identical to those of a putative nitrilase from Gibberella moniliformis (anamorph Fusarium verticillioides) [15]. A highly homologous gene was also found in Gibberella zeae (85% identities and 91% positives). The multiple sequence alignment of fungal nitrilases and related enzymes (prokaryotic nitrilases and cyanide (di)hydratases) and the level of homology between these proteins are shown in Figures 14.1 and 14.2, respectively. The fungal nitrilases display on average 51% similarity with each other, but the median is only 35%. Conserved sequences were found in all proteins, with the N- or Cterminal sequences showing low similarities throughout the set of enzymes examined, as expected. It is notable that the new nitrilases from F. solani O1 (presumably highly homologous with the nitrilases from G. moniliformis and G. zeae) and A. niger K10 are relatively distant from each other in evolution (with only about 36% homology). On the other hand, a higher homology level of 53–69% was observed between the nitrilase from strain K10 and the cyanide hydratases from Gloeocercospora sorghi [16, 17], F. solani [18], Fusarium lateritium [19], and Leptosphaeria maculans [20]. Cyanide hydratases show a high specifity for HCN, which they transform to formamide. Nevertheless, minor activities for organic nitriles were reported for the enzymes from F. lateritium (0.02–0.4% of the HCN activity) and Fusarium oxysporum (0.05% of the HCN activity). Cyanide hydratases have been reported in filamentous fungi exclusively and form a much more closely related group of enzymes than nitrilases (see [6] for a review). Cyanide dihydratases are also HCN specific, but differ in the reaction mechanism, which leads to formate. Their phylogenetic distribution is also different: these enzymes occur in bacteria, for instance, Bacillus pumilus [21] or Pseudomonas

Figure 14.1 Alignment of amino acid sequences of fungal nitrilases and related proteins (using ClustalW software). A nig, A. niger CBS 513.88 (putative nitrilase, XP_001389844); A nig K10, A. niger K 10 (aromatic nitrilase, ABX75546); A fum1, A. fumigatus Af293 (putative cyanide hydrolase/nitrilase, XP_747028); A fum2, A. fumigatus Af293 (putative nitrilase, XP_ 756085); G mon, G. moniliformis (putative nitrilase,

ABF83489); G zeae, G. zeae PH-1 (hypothetical protein, XP_386656); A fac, Acidovorax facilis 72W (nitrilase, ABD98457); R rh K22, R. rhodochrous K22 (aliphatic nitrilase, BAA02127); R rh J1, R. rhodochrous J1 (aliphatic nitrilase, Q03217); F lat, F. lateritium (cyanide hydratase, AAA33336); F sol, F. solani (cyanide hydratase, CAC69666); G sor – G. sorghi (cyanide hydratase, P32964); B pum, B. pumilus C1 (cyanide dihydratase, AAN77003).

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Figure 14.2 Homology analysis of amino acid sequences of fungal nitrilases and related proteins (see Figure 14.1) using BioEdit software.

stutzeri [22], from which they have been purified. They show a lower level of homology with the nitrilase from A. niger (for instance, about 29% for the enzymes from B. pumilus) than cyanide hydratases. The similarity of fungal nitrilases to plant nitrilases or to the Nit domain of the NitFHit protein from Caenorhabditis elegans, one of the crystallized members of the nitrilase superfamily, is below 30%, but the sequences flanking the catalytic triad (Glu–Lys–Cys) residues show higher similarities (see Figure 14.3). Among prokaryotic enzymes, the highest sequence similarities with the nitrilases from A. niger and G. moniliformis were found for those from Rhodococcus rhodochrous J1 and Acidovorax facilis 72W (34.1 and 35.5%). The above results confirm structural differences in fungal nitrilases from those in other organisms as well as among each other and encourage scientists to search for new enzymes with beneficial biocatalytic properties in filamentous fungi. 14.2.2 Selection and Screening of Nitrilase Activity

Most of the well-characterized nitrilases, which are largely of bacterial origin, have been obtained by selection methods allowing only the positive strains to grow on a medium with a nitrile as the sole nitrogen source. The same approach provided nitrile-hydrolyzing fungal isolates belonging to the genus Fusarium, among them F. solani O1, from environmental samples. 3-Cyanopyridine was used as the sole nitrogen source, while rose Bengal inhibited the growth of contaminating bacteria [23]. Utilization of 3-cyanopyridine for mycelial growth was also the criterion used for identification of nitrilase-producing strains among collection strains of filamentous fungi. Out of approximately 100 strains, 13 strains belonging to the genera Aspergillus, Fusarium, Penicillium, and Talaromyces [24] grew on this

14.2 Distribution and Evolutionary Relationship of Fungal Nitrilases

Figure 14.3 Alignment of sequences flanking the catalytic triad Cys–Glu–Lys (using ClustalW software). A nig, A nig K10, A fum1, A fum2, G mon, G zeae, A fac, R rh K22, R rh J1, F lat, F sol, G sor, and B pum – see Figure 14.1. P fluo, P. fluorescens (nitrilase, Q5EG61); K oza – Klebsiella pneumoniae subsp. ozaenae (nitrilase, P10045); S cer, Saccharomyces cerevisiae (probable hydrolase NIT3, P49954), At nit1, Arabidopsis thaliana (nitrilase 1, P32961); At nit2, A. thaliana (nitrilase 2, NP_190016); At

nit3, A. thaliana (nitrilase 3, NP_190018); At nit4, A. thaliana (nitrilase 4, NP_197622); C ele, C. elegans (hypothetical protein/CN hydrolase, CAA84681); A sp – Agrobacterium sp. KNK712 (N-carbamoyl-D-amino acid hydrolase (D-Nalpha-carbamylase), P60327); H pyl, Helicobacter pylori J99 (Formamidase, Q9ZJY8); P hor, Pyrococcus horikoshii OT3 (hypothetical protein PH0642, NP_142600). a Proteins, the crystal structures of which were determined.

medium, but considerable levels of nitrilase activity were only found in one strain of A. niger, two strains of F. oxysporum, and one strain of Penicillium multicolor [25]. The expression of nitrilase may be enhanced and, thus, screening for its activity facilitated by the choice of an appropriate nitrilase inducer. For instance, in five strains belonging to three fungal genera 2-cyanopyridine enhanced the nitrilase activity by two to three orders of magnitude. Therefore, this nitrilase inducer appears to be very efficient and of wide use in filamentous fungi [25]. Wide distribution of nitrile-hydrolyzing enzymes (presumably at least partially nitrilases) among different genera of filamentous fungi was suggested by another study [26], the primary aim of which was to examine the distribution of aldoxime dehydratases in microorganisms including 102 fungal strains belonging to 37 genera. Owing to simultaneous expression of nitrile-hydrolyzing enzymes along with aldoxime dehydratases, hydrolytic activities towards 2-phenylacetonitrile and

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3-cyanopyridine were detected in 31 and eight strains, respectively, albeit largely at low levels. Obviously, the results of the screening are strongly dependent on the substrate. Out of the total positive strains, eight and four strains belonged to the genera Fusarium and Aspergillus, respectively, but nitrile-hydrolyzing enzymes were also found in the genera Schizophyllum, Rhizopus, Talaromyces, Mortierella, Flammulina, Pycnoporus, Keratinomyces, Mucor, Coprinus, Cunninghamella, and Phycomyces, in which no nitrilase or nitrile hydratase genes have been sequenced.

14.3 Structural Properties

In general, nitrilases contain a single type of subunit, the molecular mass of which has been assessed at 32–45 kDa in a number of purified enzymes (for a review see [5, 6]). The size of subunits in fungal nitrilases from A. niger K10, F. solani O1, and F. oxysporum f. sp. melonis is in this range (see Table 14.1). In this context, the 76-kDa size reported for the nitrilase subunit in F. solani IMI196840 [30] is exceptional [10]. Typically, native nitrilases are oligomers consisting of two to about 16 identical subunits, while active monomers are rare (for a review see [5, 6]). Fungal enzymes belong to the largest molecules among nitrilases (see Table 14.1), the molecular weight of those from F. oxysporum f. sp. melonis, F. solani IMI196840, F. solani O1, and A. niger K10 having been assessed at 550, 620, 580, and >650 kDa, respectively. Smaller oligomers can retain activity, as demonstrated for the enzyme from F. oxysporum f. sp. melonis, which was resolved by non-denaturing polyacrylamide gel electrophoresis into active bands having molecular weights of 170–880 kDa with increments of 70 kDa, that is into protein species made of four to 26 subunits [11]. Co-purification of nitrilases with chaperonin proteins, which was observed for enzymes from Pseudomonas fluorescens DSM 7155 [31] and B. pallidus Dac521 [32], was recently also reported for the nitrilase from A. niger K10 [14]. According to Nterminal amino acid sequencing, the proteins co-purified with nitrilases in P. fluorescens and B. pallidus were found to exhibit a high homology with Cpn60 and GroEL proteins, respectively. The protein co-purified with the nitrilase from A. niger was probably an eukaryotic equivalent of these proteins, its N-terminal amino acid sequence showing high similarity with that of Hsp60 chaperonine polypeptide. The chaperonins may play role in assemblying subunits into a multimeric structure or in protein stabilization. Several nitrilase and cyanide dihydratase holoenzymes share the ability to assemble into supramolecular species, which were characterized as helical filaments (see Figure 14.4). This unique property was revealed by electron microscopy selected as a method of choice for the study of high-molecular weight holoenzymes. The nitrilase from F. solani formed helical rods up to 500 nm long or aggregates (see Figure 14.4a and b). Long rods were also observed in samples of the A. niger nitrilase, which, however, differed in structure and length (up to 250 nm) from those of the F. solani enzyme [15]. In addition, the enzyme from

14.3 Structural Properties

235

Table 14.1 Subunit and holoenzyme molecular masses of nitrilases from filamentous fungi : comparison with selected prokaryotic nitrilases, cyanide hydratases and cyanide dihydratase.

Enzyme

Nitrilase

Cyanide hydratase Cyanide dihydratase

Organism

A. niger K10 A. niger CBS 513.88a F. oxysporum f. sp. melonis F. solani IMI196840 F. solani O1 G. moniliformisa,b R. rhodochrous J1 R. rhodochrous K22 G. sorghi F. solani B. pumilus C1

Molecular mass (kDa) Subunit experimental

Predictedc

Holoenzyme

38.5 – 37 76 40 nd 40 41 45 45 37

40.2 40.0 – – – 36.0 40.2 42.3 40.9 40.8 37.3

> 650 – 550 (170–880d) 620 580 – 480e 650 > 300 > 300 417

Note: no entry indicates no data available. a Putative nitrilases. b Presumably highly homologous to the enzyme from F. solani O1 [15]. c Calculated by BioEdit sowftware. d Oligomers of different size produced by non-denaturing electrophoresis [11]. e The 80-kDa dimer is inactive [29].

A. niger seemed to form no aggregates (see Figure 14.4c and d). The cyanide dihydratase from B. pumilus formed left-handed helical rods at pH 5.4 and short spirals at pH 8.0 [33]. The long helices were also found in samples of the bacterial nitrilase from R. rhodochrous J1 [29]. In its native form (480 kDa) this enzyme formed ‘c’shaped particles, but after 1 month of storage at 4 °C the cleavage of approximately 39 amino acids from the C-terminus led to the formation of active clusters with a molecular weight of >1500 kDa, which were observed under electron microscopy as left-handed helical rods of different length. The same forms were detected in mutants with C-terminus deletions. The C-terminal amino acids sequence seems to make the formation of helical filaments in this nitrilase impossible. The lack of crystallography data prevented gaining a deep insight into the structure and function of nitrilases, but a considerable homology level between the active sites of these enzymes and proteins, the structures of which have been solved (see Figure 14.3), can be helpful in nitrilase modeling, as demonstrated for the enzyme from R. rhodochrous J1 [29]. The members of the nitrilase superfamily are typically α/β proteins with a four-layer αββα sandwich architecture. These units associate in a hexamer of formamidase from Helicobacter pylori [34], tetramers of NitFhit protein from C. elegans [35], and N-carbamoyl-D-amino acid amidohydrolase from Agrobacterium sp. strain KNK712 [36] or dimers of hypothetical protein from Pyrococcus horikoshii OT3 [37] and CN hydrolase from Saccharomyces cerevisiae [38].

Reference

[14] – [11] [10] [15] – [27] [28] [16] [18] [21]

236


Figure 14.4 Electron micrographs of negatively stained nitrilases from (a and b) F. solani O1 and (c and d) A. niger K10. The scale bar represents 100 nm. From [15] with permission.

14.4 Catalytic Properties 14.4.1 Reaction Mechanism

The hypothetical nitrilase catalytic mechanism based on previous schemes (for a review see [6]), which has been recently refined to bring more understanding to the formation of two possible end-products [39], is discussed in Chapter 16. According to a generally accepted hypothesis, the catalytic mechanism involves a nucleophilic attack on the cyano carbon by the sulfhydryl group of the conserved

14.4 Catalytic Properties

237

Table 14.2 Ratio of amide : acid produced by nitrilases from different organisms.

Organism

Amide : Acid ratio (substrate)a

Reference

A. thalianab

19 (3-nitroacrylonitrile) 13.3 (3-cyanoacrylonitrile) ≈4.5–5.7 (α-fluoroarylacetonitriles) ≈1.5 (β-cyano-L-alanine) 5.2 (2-cyanopyridine) 1.1 (4-chlorobenzonitrile) 0.5 (4-cyanopyridine) 0.4 (1,4-dicyanobenzene) 0.04–0.06 (benzonitrile, propionitrile, acrylonitrile) 0.03 (3-cyanopyridine, 4-cyanopyridine) 0.02 (benzonitrile) ≈0.1 (N-methyl-3-cyano-4-methoxy-2-pyridone) 10 (R,S)-2-chloro-2-phenylacetonitrile 1.7 (S)-O-acetylmandelonitriled 1.25 (S)-mandelonitriled 0.5 (R)-O-acetylmandelonitriled 0.12 (R)-mandelonitriled 0.02 2-phenylacetonitrile

[41] – – [42] [14] – – – [11] [15] – [43] [39, 44] – – – – [45]

A. thalianac A. niger K10

F. oxysporum f. sp. melonis F. solani O1 Pseudomonas sp. P. fluorescens EBC 191

Rhodococcus sp. ATCC39484

a Only substrates from which the maximum amide amounts were produced are shown for each enzyme. b AtNIT1 enzyme. c AtNIT4 enzyme. d At pH 6 (amide : acid ratio pH-dependent).

cysteine, leading to a covalent enzyme–thioimidate complex that is hydrolyzed to give a tetrahedral intermediate. Two pathways of intermediate decomposition involve either ammonia or enzyme as the leaving group, thus releasing acylenzyme or amide, respectively (see Figure 16.10). Data from experiments with bacterial and plant nitrilases have indicated the importance of the electronic or steric effect of the substrate, amides being preferably formed from nitriles with electron-withdrawing or bulky substituents [40–45]. Reactions catalyzed by fungal nitrilases support this view (see Table 14.2). The nitrilase from A. niger K10 belongs to enzymes that are very prone to amide formation, as it forms high amide/acid molar ratios from nitriles with an electron-withdrawing heteroatom or substituent (up to about 5 : 1 for 2-cyanopyridine) [14]. The nitrilase from F. solani O1 forms only low amounts of amide by-products (≤3% of the total product) [15]. This is in agreement with amide production by the nitrilase from F. oxysporum, which produced 4–6% of amides from some nitriles [11]. The chemoselectivity of the third purified nitrilase from the genus Fusarium is unknown as the enzyme assay was based on ammonia determination [10].

238


14.4.2 Substrate Specificity

All known fungal nitrilases exhibit high relative activities towards benzonitrile and its m- and p-substituted derivatives. Therefore, according to the nitrilase classification [46] they belong to aromatic nitrilases. However, almost all fungal nitrilases also hydrolyze aliphatic nitriles, albeit at lower relative rates. The substrate specificities of all four biochemically characterized fungal nitrilases and two enzymes from rhodococci (an aromatic and an aliphatic nitrilase) are compared in Table 14.3. Only a few nitriles were examined as substrates of all these enzymes. Moreover, the comparison of substrate specificity is difficult in some cases as different activity assays have been used for different enzymes. For instance, the determination of activity by ammonia measurement did not reflect potential amide formation. Nevertheless, common patterns can be found in substrate preferences of aromatic nitrilases. In general, substitution of the benzene ring decreases the reactivity of the compounds with a few exceptions. Halobenzonitriles are generally among the best substrates due to a strong electron-withdrawing effect of the substituents, which is favorable for the nucleophilic attack of the enzyme on the cyano group. On the other hand, the derivatives with electron-donating groups (3-hydroxybenzonitrile and tolunitriles) are largely hydrolyzed at low relative rates, with a few exceptions. For instance, the nitrilases from F. solani O1 [15] and R. rhodochrous J1 [27] have high relative activities for 3-hydroxybenzonitrile and tolunitriles, respectively. Differences between the reactivities of meta- and para-isomers also seem to reflect the electron densities on the cyano group [14]. For instance, a lower electron density may be expected on the cyano group of 1,4-dicyanobenzene in comparison with 1,3-dicyanobenzene and the much greater reactivity of the former compound is in accordance with this expectation. Nitrilases are largely very sensitive to steric hindrances by the substituent or heteroatom at the ortho-position, 2-substituted benzonitriles or 2-cyanopyridine being rarely hydrolyzed at acceptable rates. 4-Cyanopyridine is a better substrate than 3-cyanopyridine for all the examined nitrilases. The difference between the reactivities of these compounds can also be rationalized by a lower electron density on the cyano group of 4-cyanopyridine. 2,4- and 2,6-Pyridinedicarbonitrile are also accepted as substrates by nitrilases from A. niger K10 and F. solani O1, the major products being the corresponding cyanocarboxylic acids [47]. Among aliphatic nitriles, unbranched saturatured or unsaturated compounds with a medium chain length (for instance, propionitrile, butyronitrile, hexanenitrile, and acrylonitrile) are hydrolyzed with the highest relative rate (about 20–35% of that for benzonitrile) by aromatic nitrilases. Branched nitriles are in general poor substrates of these enzymes but, for instance, isobutyronitrile is one of the best substrates of the aliphatic nitrilase from R. rhodochrous K22 [28].

14.4 Catalytic Properties

239

Table 14.3 Substrate specificity of purified nitrilases from filamentous fungi : comparison with prokaryotic

nitrilases. Substratea

Benzonitrile 3-Hydroxybenzonitrile 3-Fluorobenzonitrile 4-Fluorobenzonitrile 3-Chlorobenzonitrile 4-Chlorobenzonitrile 3-Bromobenzonitrile 4-Bromobenzonitrile 3-Nitrobenzonitrile 4-Nitrobenzonitrile 3-Tolunitrile 4-Tolunitrile 1,4-Benzodinitrile 3-Cyanopyridine 4-Cyanopyridine 2-Phenylacetonitrile Crotononitrile Acetonitrile Propionitrile Butyronitrile Isobutyronitrile Valeronitrile Hexanenitrile Adiponitrilee Acrylonitrile Methacrylonitrile Allylonitrile 2-Thiophenacetonitrile

Relative rateb A. niger K10c

F. oxysporum f. sp. melonisc

F. solani IMI196840d

F. solani O1c

R. rhodochrous J1c

R. rhodochrous K22c

100 5.8 – – 41.0 29.8 – – – – 5.5 3.4 79.5 32.4 410.7 10.8 – 0 6.9 17.6 0 19.6 – – – – – 56.1

100 0 – – – – – – – – 0 9.0 – 25 – 8 17 0.3 20 33 15 – 24 11 35 19 34 –

100 21.0 74.3 150.2 40.4 85.6 34.4 37.6 7.0 27.6 17.9 9.9 213.3 31.0 124.7 0 – – – – – – – – – – – –

100 80 – – 87 40 – – – – 33 16 – 28 130 – – – 18 20 4 26 – – – 14 – –

100 22.2 24.8 26.2 137 114 26.3 23.6 74.2 74.2 99.6 116 – 22.1 25.6 1.8 1.7/11.5e – – – – – – – 0/58.8e – – 10.7

27.1 – – – – – – – 74.5 – – – – 7.3 9.7 27.3 100 28.3 12.7 18 271 40.8 – 110f 348 143 – 73.5g

Note: no entry indicates no data available. a This table shows only those substrates that were transformed by at least one of the nitrilases at ≥20% of the reaction rate for the reference substrate (crotononitrile for R. rhodochrous K22 and benzonitrile for other organisms). b The relative activities of enzymes from F. oxysporum f. sp. melonis, F. solani IMI196840, R rhodochrous J1, and R. rhodochrous K22 have been taken from the previous review [6], while those of enzymes from A. niger K10 and F. solani O1 were taken from works [14] and [15], respectively. c Relative rates at a given substrate concentration are compared to that for benzonitrile. d Maximum reaction velocities (Vmax) are compared to that of benzonitrile. e Activity for crotononitrile and acrylonitrile was recovered by subunit aggregation in presence of 10% saturated ammonium sulfate. f The enzyme also hydrolyzed other dinitriles, that is glutaronitrile, succinonitrile, fumaronitrile, malononitrile, and sebaconitrile at relative rates of 345, 271, 27.4, 45.1, and 15.7%. g The enzyme also hydrolyzed 3-thiophenacetonitrile at a relative rate of 66.6%.

240


14.4.3 Activity and Stability

Out of the three purified nitrilases from the genus Fusarium, two enzymes have high specific activities towards benzonitrile (143 and 156 U/mg of protein), the nitrilase from F. solani IMI196840 being exceptional regarding not only its subunit size (see Section 14.3) but also its specific activity, which is two orders of magnitude lower (see Table 14.4). The specific activity of the nitrilase from A. niger (91.6 U/mg of protein) is probably underestimated, the enzyme having not yet been purified to homogeneity. Fungal cyanide hydratases are also characterized by high specific activities, which may be up to one order of magnitude greater than those of fungal nitrilases [17] and, in general, also markedly higher than those of bacterial cyanide dihydratases (see Table 14.4). The specific activites of bacterial nitrilases are generally lower than those of fungal nitrilases, as illustrated by two examples of enzymes from rhodococci in Table 14.4. The nitrilase from F. solani IMI196840 also differs from other fungal nitrilases in terms of its Km value for benzonitrile (0.039 mM) [10], which is markedly lower than those of other fungal nitrilases (0.2–1.5 mM) [11, 14, 15]. In general, even higher Km-values, that is in the range of 2.6–12 mM for wild-type enzymes and 5.9–90 mM for hexahistidine-tagged enzymes, have been determined for cyanide hydratases/dihydratases [17]. Typically, nitrilases exhibit the highest activities at neutral or slightly alkaline pH values except for the aliphatic nitrilase from R. rhodochrous K22 with a slightly acidic pH optimum (for a review see [5]; Table 14.4). Nevertheless, examination of the activity profiles of fungal nitrilases revealed some differences among these enzymes. Contrary to the nitrilase from F. oxysporum f. sp. melonis, which was highly active irrespective of pH value in the range of pH 6–11, the nitrilases from F. solani O1 and A. niger K10 required a relatively narrow range of neutral and slightly alkaline pH values. In general, similar conditions are also beneficial for nitrilase stability (see Table 14.4). The temperature optima of fungal nitrilases (see Table 14.4) are similar to those of bacterial nitrilases, which are largely highly active at 40–50 °C except for the rare nitrilases from moderate thermophiles (for a review see [5]) and the highly thermostable nitrilase from Pyrococcus abyssi [48]. Apparently, there is no marked difference between the fungal nitrilases in this respect, though accurate comparison is not possible due to the different protocols used. The enzyme from A. niger K10 was fairly stable at 30 °C with a half-life of 11 h, but its stability decreased sharply at 35 and 40 °C, as indicated by enzyme half-lives of 6.2 and 2.8 h, respectively [14]. The half-life of the purified nitrilase from F. solani O1 was about 20 and 11 h at 35 and 45 °C, respectively, indicating higher stability of this enzyme (unpublished results). In order to assess the potential applications of these new nitrilases in biocatalytic processes, data on their operational stability were required. To this end, we investigated the kinetic behavior of enzymes from F. solani and A. niger either immobilized on solid supports or retained in stirred ultrafiltration membrane reactors in continuous experiments.

Table 14.4 Specific activity, reaction optima, and stability of purified nitrilases from filamentous fungi: comparison with prokaryotic nitrilases, cyanide hydratase, and cyanide dihydratase.

Enzyme

Aromatic nitrilase

Aliphatic nitrilase Cyanide hydratase Cyanide dihydratase

Organism

A. niger K10 F. solani f. sp. melonis F. solani IMI196840 F. solani O1 R. rhodochrous J1 R. rhodochrous K22 G. sorghi B. pumilus C1

91.6 (benzonitrile) 143 (benzonitrile) 1.66 (benzonitrile) 156.0 (benzonitrile) 15.9 (benzonitrile) 0.737 (crotononitrile) 555 (KCN) 50.9 (KCN)

Optimum

Stability

Reference

pH

T (°C)

pH

T (°C)

8 6–11 7.8–9.1 7–9 7.6 5.5 7.8 7.8–8.0

45 40 50 40–45 45 50 – 37

7.2–9.0 6–11 – 6–8a – 6–8 – –

≤ 30 ≤ 40 – ≤ 35a ≤ 45 ≤ 40 23 –

[14] [11] [10] [15] [27] [28] [16, 17] [17, 21] 14.4 Catalytic Properties

Note: no entry indicates no data available. a Unpublished results.

Specific activity (U/mg protein)

241

242


The nitrilases were immobilized with 70–95% activity recovery on hydrophobic carriers (for instance, Butyl Sepharose) packed in columns [49–51]. The benefit of this immobilization method is its simplicity and quickness, as well as the possibility of compensating biocatalyst deactivation by reloading the column with fresh enzyme. On the other hand, the high salt concentration (for instance, 0.8 M ammonium sulfate), which is necessary for enzyme binding onto the carrier, complicates the purification of the reaction products. This is avoided by using ion exchangers, for instance Q Sepharose [49], as the solid supports, but this method is not compatible with high concentrations of substrates, as the corresponding products (acids) interfere with enzyme binding. Fungal nitrilases immobilized on these columns were applicable to continuous biotransformation of heteroaromatic nitriles like 3- and 4-cyanopyridine, the products of which, nicotinic and isonicotinic acid, respectively, are of commercial interest. The enzyme from F. solani exhibited a higher stability than that from A. niger at 35 °C. The conversion of 3-cyanopyridine by the former enzyme was nearly quantitative within 24 h [50], while it decreased by 30% within 15 h in the case of the latter [49]. Similar differences in operational stabilities were observed during conversion of 4-cyanopyridine. The stability of the enzymes depended on the substrate used, both nitrilases being more stable during the conversion of 4cyanopyridine in comparison with 3-cyanopyridine. Continuous stirred membrane reactors proved to be useful in laboratory-scale studies of continuous biocatalytic processes based on reactions catalyzed by nitrile hydratase and amidase (see Chapter 17). The retention of enzymes in ultrafiltration membrane reactors can be regarded as a special type of immobilization with a number of advantages resulting from the absence of enzyme–carrier interactions. At temperatures below the reaction optimum the enzymes from both F. solani and A. niger showed fair stability during long-term runs in ultrafiltration membrane reactors. Similarly, as in the above experiments, however, the former enzyme was more stable. During conversions of 50 mM of 4-cyanopyridine, A. niger showed a half-life of 50 h at 30 °C (unpublished results) and F. solani a halflife of 277 h at 35 °C [15]. However, as a very small amount of the enzyme (99) 44 (2)

71 (5) 14 (51) 0

26 (78) 54 (65) 13 (>99)

47 (8) 48 (6) 0

84 (0) 10 (11) 0

5b 0 95b

a Percentage isolated yield after chromatographic purification with percentage e.e. in parentheses. b The e.e. was not determined. Table 15.2 Rhodococcus erythropolis NCIMB 11540 whole cell biohydrolysis: isolated products.

Nitrilea Amidea Acida

(±)-1a

(±)-2a

(±)-3a

(±)-4a

(±)-6a

(±)-10a

0 13 (>99) 86 (5)

50 (16) 49 (15) 0

24 (98) 56 (59) 15 (97)

50 (10) 41 (8) 0

91 (1) 2 (32) 0

0 0 100

a Percentage isolated yield after chromatographic purification with percentage e.e. in parentheses. Table 15.3 Rhodococcus sp. R312 whole cell biohydrolysis: isolated products.

Nitrilea Amidea Acida

(±)-1a

(±)-2a

(±)-3a

(±)-4a

(±)-6a

(±)-10a

46 (30) 10 (>99) 34 (14)

11 (51) 75 (7) 0

33 (47) 42 (77) 16 (87)

44 (10) 43 (4) 0

73 (0) 25 (6) 0

9b 8b 83b

a Percentage isolated yield after chromatographic purification with percentage e.e. in parentheses. b The e.e. was not determined.

run at standardized conditions (amount of wet cell weight per amount of substrate, substrate concentration, and so on) and were stopped after 24 h in order to compare the particular product distribution [34]. Generally, the five-ring amino nitriles reacted significantly faster than the six-ring compounds. Notably, the relative configuration of the ring substituents had a distinct impact on the product selectivity of the whole cell transformations: trans-2-aminocyclopentane carboxylic acid and trans-2-aminocyclohexanecarboxylic acid could be synthesized from the respective amino nitriles trans-(±)-1a and trans-(±)-3a exclusively. Whereas trans-2-amino nitriles reacted smoothly to acids and amides, accumulating the acid with the preceding reaction time, the cis-amino nitriles (±)-2a and (±)-4a reacted more slowly to the amides. A similar diastereodifferentiation was also observed for structurally analogous carbocyclic β-hydroxy nitriles.

251

252

15 Nitrilase- and Nitrile Hydratase-catalyzed Enantioselective Preparation

Figure 15.3 Transformation of (±)-2a by R. erythropolis A4 with dependence on the reaction time; 䊉 nitrile, 䊏 amide, and 䉱 acid.

Surprisingly, acyclic vinylogous amino nitrile (±)-6a, prepared by the Aza–Baylis–Hillman reaction, only reacted to the corresponding α-methylene-βamino amide [34]. In agreement with this, the related oxo-analog was recently reported to give the amide as well upon incubation with Rhodococcus sp. AJ270 cells [35]. The enantioselective synthesis of 2-phenylisoserine derivatives is an attractive goal and several attempts, such as the application of acylase [36], lipase [37], and, more recently, bakers yeast reductase enzymes [38], were made to prepare the taxol side chain (see Section 15.1). No nitrile-converting enzymes have been investigated so far. The chemical instability of 2-hydroxy-3-amino-3-phenylpropionitrile (a cyanohydrin) in aqueous solution afforded the preparation of the cyclic protection products trans-(±)-8a and cis-(±)-9a (Figure 15.2). Incubation with whole cells of Rhodococcus sp. R 312 yielded (i.e. 50%) the amide trans-8b in 98% e.e. quantitatively, with not even traces of the acid, leaving the cis-nitrile untouched [34]. Acyclic amino nitriles (±)-5a and (±)-7a reacted sluggishly and no clear trend in the formation of a preferred product was observed, thus their results have been omitted. N-Carbobenzoxylated pyrrolidine-3-carbonitrile, a precursor for β-proline, was the fastest reacting five-membered compound. The substrate was entirely consumed within a few minutes yielding β-proline in low enantiomeric purity [39]. The hydrolysis of cis-2-NHTs-cyclopentane nitrile (±)-2a over a prolonged reaction time was investigated to check whether it could be extended over the amide stage to formation of acid 2c, a derivative of cispentacin. The conversion–time diagram of (±)-2a is depicted in Figure 15.3, suggesting that the acid 2c could be accumulated in an amount of 34% after 206 h [34]. Similar to diastereoselectivities, the enantioselectivities also strongly correlate with the relative stereochemistry of the 1,2-substituents and with the ring size of amino nitriles (±)-1a–(±)-4a: trans-cyclopentane carbonitriles gave the amides in excellent enantiopurity, whereas trans-cyclohexane carbonitriles resulted in the

15.3 Nitrilase Biotransformations

Figure 15.4 Enantiomeric excess of 2a–2c with dependence on conversion by R. erythropolis A4; 䊉 nitrile, 䊏 amide, and 䉱 acid.

formation of the acids in excellent enantiopurities, though at lower conversion. The e.e.s of cis-compounds, however, were lower throughout [34]. Thus, the trans-aminocyclohexanecarboxylic acid 3c was prepared by all three microorganisms in very high e.e. (87–99%), whereas the intermediate amides were formed in moderate e.e. In contrast, five-membered trans-amide 1b was obtained in very high enantiopurity, but not so its acid [34]. The high enantiomeric purity of the remaining trans-nitriles 1a and 3a is remarkable in view of the extent of conversion (38 and 24%, respectively).This can be attributed to the reportedly high enantioselectivity of the nitrile hydratase from R. erythropolis A4 for the transformation of (±)-1a to 1b at the least [40]. The conversion of cis-2-NHTs-cyclopentane carbonitrile (±)-2a to amide 2b and acid 2c is plotted versus their e.e.s in Figure 15.4. The transformation was monitored over a reaction time of 206 h. The curve suggests the typical enantiopurity conversion dependence expected from a kinetic resolution. This is in contrast to the results in Tables 15.1–15.3, where no acid was formed within the reaction time of 24 h: 33% acid could be obtained in 34% e.e. after 206 h [34]. Unfortunately, the e.e. of the acid did not exceed 56%, even at low conversion. The reasons for that are not clear.

15.3 Nitrilase Biotransformations 15.3.1 Enantioselective Hydrolysis of β-Amino Nitriles

The availability of ready for use nitrilase preparations [41] with the advantage of avoiding the laborious handling of whole cell biotransformation systems has simplified the reaction protocol substantially and prompted the screening of nitrilases

253

254


Figure 15.5 Heterocyclic amino nitriles for enzymatic transformations (only one enantiomer is depicted).

Table 15.4 Nitrilase hydrolysis to heterocyclic amino acids

10c–12c.

NIT-101 NIT-104 NIT-105 NIT-106 NIT-107

(±)-10ca

(±)-11ca

(±)-12ca

56b – 44 (73) 44 (76) –

– 15 (75) – – 50 (93)

– – – 5b –

a Percentage conversion determined by HPLC with the percentage e.e. in parentheses. b The e.e. was not determined.

with regard to novel substrates. Surprisingly, none of the carbocyclic β-amino nitriles (±)-1a–(±)-4a depicted in Figure 15.2 were accepted by the enzymes [39]. This situation changed when the nitrogen atom was incorporated into the ring, as in case of the N-heterocyclic nitriles (±)-10a–(±)-12a shown in Figure 15.5, which were found to be highly applicable substrates. The results from the transformations are given in Table 15.4 [42]. A significantly different enzyme activity was noticed here depending on the ring size. Five-membered pyrrolidine-3-carbonitriles were formed close to their theoretical yields within a shorter reaction time (a maximum of 24 h) than sixmembered piperidine-3- and 4-carbonitriles, which required transformation times within days. Nevertheless, the enzyme activities remained nearly unchanged over this time period. Nipecotic acid 11c could be prepared and isolated in 93% e.e., thus allowing an efficient and decisively short enantioselective synthesis of this heterocyclic amino acid [42], compared to existing literature procedures (Section 15.1). N-Toluenesulfonyl protected acids were formed in enantioselectivities superior to N-carbobenzoxylated derivatives, a fact also observed throughout the work on carbocyclic γ-amino acids. Although some product from (±)-12a was formed by NIT-106, pipecolic acid 12c could not be synthesized in a preparatively satisfying way, in particular because the amide formation is twice as high as the acid formation. A structural comparison of heterocyclic amino nitriles (±)-10a–(±)-12a with carbocyclic β-amino nitriles


Figure 15.6 Novel (±)-γ-amino nitriles for enzymatic transformations to enantioenriched γ-amino carboxylic acids (only one enantiomer is depicted).

(±)-1a–(±)-4a provides an explanation for the differing reactivities of nitrilases versus nitrile hydratases with regard to these compounds. None of them, carbocyclic substrates (±)-1a–(±)-4a and pipecolic carbonitrile (±)-12a, react with nitrilases. This can likely be attributed to a sterically unfavorable 1,2-substitution pattern in the carbocycle and the piperidine-2-carbonitrile (±)-12a, where the protecting group of the ring nitrogen atom is taking up a similar position to the 1,2carbocyclic substituents, thus resulting in substantial hindrance for the enzyme. On the other hand, nitriles (±)-10a and (±)-11a are structurally more closely related to γ-amino nitriles (±)-13a–(±)-16a (Section 15.3.2 and Figure 15.6) where the protecting group attached to the ring nitrogen atom represents the exocyclic amino substituent in the 3-position of the carbocycles. The carbocyclic γ-amino nitriles (±)-13a–(±)-16a were found to react consistently in excellent manner with the nitrilases throughout (see Section 15.3.2 and Figure 15.6). Not surprisingly, some nitrilase reactions were accompanied by the formation of the corresponding amides, such as pipecolic amide 12b (up to 10%) and pyrrolidine-3-carboxamide 10b (for a discussion of nitrile hydratase activity of nitrilases see Section 15.3.3). 15.3.2 Enantioselective Hydrolysis of γ-Amino Nitriles

During our work on enzymatic nitrile transformation, carbocyclic γ-amino nitriles have emerged from a comprehensive screening among structurally diverse amino nitriles as being well-suited substrates for nitrilase-mediated hydrolysis. In contrast, as stated in the previous section, the analogous carbocyclic β-amino nitriles are strictly non-substrates for nitrilases. For reasons already outlined in the introduction (Section 15.1), we were especially interested in the enantioselective synthesis of cyclic γ-amino acids serving as conformational GABA mimics in neuroreceptor research. The structures of amino nitriles (±)-13a–(±)-16a are depicted in Figure 15.6 [43]. All nitrilases examined [41] that exhibited sufficient activity from a screening [43] turned out to be sensitive towards changes in the ring size and relative configuration of the 1,3-substituents. The results of preparative transformations are shown in Table 15.5. Generally, cyclopentane carbonitriles reacted much faster

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15 Nitrilase- and Nitrile Hydratase-catalyzed Enantioselective Preparation Table 15.5 Synthesis of enantioenriched γ-amino acids 13c–16c from (±)-γ-amino nitriles.

NIT-106 NIT-107

(±)-13ca

(±)-14ca

(±)-15ca

(±)-16ca

45 (97) –

36 (55) –

29 (99) –

– 46 (86)

a Percentage isolated yield after chromatographic purification with the percentage e.e. determined by HPLC in parentheses.

Figure 15.7 Conversion–time diagram of cis-NHTscyclopentane carbonitrile ((±)-13a) hydrolysis by NIT-106.

than the cyclohexane carbonitriles. The cis-acids (+)- or (−)-13c and (+)- or (−)-15c are formed in excellent e.e. (97 and 99%, respectively) but not so the corresponding trans-acids 14c and 16c [43]. Essentially, nitrilases NIT-106 and NIT-107 were the most efficient catalysts, whereas NIT-101 and NIT-105 turned out to be less suitable for the synthesis of γ-amino acids. cis-3-Aminocyclopentanecarboxylic acid (+)-13c was prepared by NIT-104, whereas (−)-13c was produced by NIT-106 in an enantiocomplementary manner in a high e.e. (97%) close to the theoretical yield of a kinetic resolution. The respective trans-isomer (−)-14c was obtained in only 55% e.e. by the same enzyme. All other nitrilases examined could not enhance this result. NIT-106 revealed similar outstanding selectivities in the transformation of six-membered aminonitrile cis-(±)-15a to (−)-15c in almost optical purity (>99% e.e.) and in a 29% isolated yield [43]. The conversion–time diagram of (±)-13a to 13c in Figure 15.7 shows that the kinetic resolution of this substrate proceeds nearly quantitatively within 2 h. The enzyme’s prerequisite for both high catalytic activity and enantioselectivity is fulfilled at best by a 1,3-diequatorial position of the substituents, as present in


Figure 15.8 Structure of amidinomycin (−)-17c.

cis-(±)-13a and cis-(±)-15a (Figure 15.6). Notably, the remaining nitriles were also recovered in up to 98% e. In contrast, the respective trans-isomers 14c and 16c were obtained in good yields exclusively by NIT-107, though in diminished e.e. (86 and 74%, respectively). In this case, nitrilase NIT-106 completely failed to catalyze the transformation of trans-(±)-16a. The absolute configuration of cis-13c was assigned to be (1R,3S) by comparison of the HPLC elution order with the available reference acid cis-(1R,3S)-3-ACPA and was found to coincide serendipitously with the configuration of natural amidinomycin (Figure 15.8). The absolute configuration of cis-15c could be assigned analogously. For an assignment of the trans-amino acids 14c and 16c, the transcarbonitriles were chemically epimerized by base, though with concomitant nitrile hydrolysis, to the thermodynamically more stable 1,3-cis-isomers following the assignment by the HPLC elution order again. Interestingly, NIT-106 acted highly R-selective in contrast to all other nitrilases applied in this study, which were S-selective with regard to the ring carbon at C-1 [43]. 15.3.3 Nitrile Hydratase Activity of Nitrilases

The formation of amides during nitrilase reactions was observed as early as 1964 [44]. However, the rise of more sophisticated analytical techniques has triggered communications on this subject substantially in recent times. This ‘nitrile hydratase activity’ of nitrilases was frequently observed when substrates activated in the α-position to the nitrile group were used [45, 46] . Most recently, a study appeared on this subject suggesting a mechanistic rationale of amide formation [47]. Throughout the work on amino nitriles discussed in this chapter, amide formation was observed in agreement with the structural features mentioned above. Thus, α-activated nitrile (±)-12a (Figure 15.5), a poor substrate for all nitrilases except NIT-106, gave pipecolic amide 12b (in an amount of up to 10% even as the predominant product) and (±)-10a (Figure 15.6) unexpectedly gave pyrrolidine-3carboxamide 10b in amounts of up to 31% depending on the nitrilase employed (the 31% value was achieved by NIT-104), although its nitrile moiety is not in agreement with an α-activating position [42]. On the other hand, there was no formation of piperidine-3-carboxamide. Both protected α-hydroxy-activated amino nitriles (±)-trans-8a and (±)-cis-9a (Figure 15.2) gave rise to the amides 8b and 9b as the sole products [48]. Consistent with these findings, neither carbocyclic βamino amides nor γ-amino amides were detected at any stage of the nitrilase transformations.

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References 1 Garner, P. and Ramakanth, S. (1986) The Journal of Organic Chemistry, 51, 2609–2612. 2 Davies, S.G., Ichihara, O., Lenoir, I. and Walters, I.A.S. (1994) Journal of the Chemical Society-Perkin Transactions, 1, 1411–1415. 3 Sone, H., Nemoto, T., Ishiwata, H., Ojika, M. and Yamada, K. (1993) Tetrahedron Letters, 34, 8449–8452. 4 Juaristi, E. (ed.) (1997) Enantioselective Synthesis of β-Amino Acids, Wiley-VCH Verlag GmbH, Weinheim. 5 Denis, J.N., Correa, A. and Greene, A.E. (1990) The Journal of Organic Chemistry, 55, 1957–1959. 6 Cardillo, G. and Tomasini, C. (1996) Chemical Society Reviews, 25, 117–128. 7 Konishi, M., Nishio, M., Saitoh, K., Miyaki, T., Oki, T. and Kawaguchi, H. (1989) Journal of Antibiotics, 42, 1749–1755. 8 Gellman, S.H. (1998) Accounts of Chemical Research, 31, 173–180. 9 Cheng, R.P., Gellman, S.H. and DeGrado, W.F. (2001) Chemistry Reviews, 101, 3219–3232. 10 Juaristi, E., Quintana, D. and Escalante, J. (1994) Aldrichimica Acta, 27, 3–11. 11 Cole, D.C. (1994) Tetrahedron, 50, 9517–9582. 12 see references 1-3 in [42] 13 see references 4 in [42] 14 Mazzini, C., Lebreton, J., Alphand, V. and Furstoss, R. (1997) The Journal of Organic Chemistry, 62, 5215–5218. 15 Nazabadioko, S., Pérez, R.J., Brieva, R. and Gotor, V. (1998) Tetrahedron: Asymmetry, 9, 1597–1604 and references therein. 16 Eichhorn, E., Roduit, J.-P., Shaw, N., Heinzmann, K. and Kiener, A. (1997) Tetrahedron: Asymmetry, 8, 2533–2536 and references therein. 17 Sánchez-Sancho, F. and Herradón, B. (1998) Tetrahedron: Asymmetry, 9, 1951–1965. 18 Andersen, K.E., Sorensen, J.L., Lau, J., Lundt, B.F., Petersen, H., Huusfeldt, P.O., Suzdak, P.D. and Swedberg, M.D. (2001) Journal of Medicinal Chemistry, 44, 2152–2163 and references therein.

19 Simonyi, M. (1996) Enantiomer, 1, 403–414. 20 Chebib, M. and Johnston, G.A.R. (1999) Clinical and Experimental Pharmacology & Physiology, 26, 937–940. 21 Krogsgaard-Larsen, P., Froelund, B. and Frydenvang, K. (2000) Current Pharmaceutical Design, 6, 1193–1209. 22 Chebib, M., Duke, R.K., Allan, R.D. and Johnston, G.A.R. (2001) European Journal of Pharmacology, 430, 185–192. 23 Allan, R.D., Johnston, G.A.R. and Twitchin, B. (1981) Australian Journal of Chemistry, 34, 2231–2236. 24 Allan, R.D., Johnston, G.A.R. and Twitchin, B. (1979) Australian Journal of Chemistry, 32, 2517–2521. 25 Milewska, M.J. and Polonski, T. (1994) Tetrahedron: Asymmetry, 5, 359–362. 26 Nakamura, S., Karasawa, K., Yonehara, H., Tanaka, N. and Umezawa, H. (1961) The Journal of Antibiotics, 14 (Ser. A), 103–106. 27 Chênevert, R., Lavoie, M., Courchesne, G. and Martin, R. (1994) Chemistry Letters, 23(1), 93–96. 28 Bergmeier, S.C., Cobás, A.A. and Rapoport, H.J. (1993) Bioorganic Chemistry, 58, 2369–2376. 29 Trost, B.M., Stenkamp, D. and Pulley, S.R. (1995) Chemistry – European Journal, 1, 568–572. 30 Sung, S.-Y. and Frahm, A.W. (1996) Archiv der Pharmazie, 329, 291–300. 31 Chênevert, R. and Martin, R. (1992) Tetrahedron: Asymmetry, 3, 199–200. 32 Evans, C., McCague, R., Roberts, S.M. and Sutherland, A.G. (1991) Journal of the Chemical Society-Perkin Transactions, 1, 656–657. 33 Preiml, M., Hillmayer, K. and Klempier, N. (2003) Tetrahedron Letters, 44, 5057–5059. 34 Winkler, M., Martínková, L., Knall, A.C., Krahulec, S. and Klempier, N. (2005) Tetrahedron, 61, 4249–4260. 35 Wang, M.-X. and Wu, Y. (2003) Organic & Biomolecular Chemistry, 1, 535–540. 36 Soloshonok, V.A. (1997) Biocatalytic entry to enantiomerically pure β-amino acids, in Enantioselective Synthesis of β-Amino Acids (ed. E. Juaristi), Wiley-VCH Verlag GmbH, Weinheim, pp. 443–464.

References 37 Sih, C.J. (1997) Chemoenzymatic synthesis of the side chain of taxol, in Enantioselective Synthesis of -Amino Acids (ed. E. Juaristi), Wiley-VCH Verlag GmbH, Weinheim, pp. 433–442. 38 Feske, B.D., Kaluzna, I.A. and Steward, J.D. (2005) The Journal of Organic Chemistry, 70, 9654–9657. 39 Knall, A.C. (2005) TU Graz, Diploma Thesis. 40 Prepechalová, I., Martínková, L., Stolz, A., Ovesná, M., Bezouska, K., Kopecký, J. and Kren, V. (2001) Applied Microbiology and Biotechnology, 55, 150–156. 41 Nitrilase NIT-101-NIT-108, Codexis Inc., Pasadena, CA. 42 Winkler, M., Meischler, D. and Klempier, N. (2007) Advanced Synthesis & Catalysis, 349, 1475–1480.

43 Winkler, M., Knall, A.C., Kulterer, M.R. and Klempier, N. (2007) The Journal of Organic Chemistry, 72, 7423–7426. 44 Hook, R. and Robinson, W. (1964) The Journal of Biological Chemistry, 239, 4263–4267. 45 Effenberger, F. and Osswald, S. (2001) Tetrahedron: Asymmetry, 12, 279–285. 46 Osswald, S., Wajan, H. and Effenberger, F. (2002) European Journal of Biochemistry, 269, 680–687. 47 Fernandes, B.C.M., Mateo, C., Kiziak, C., Chmura, A., Wacker, J., van Rantwijk, F., Stolz, A. and Sheldon, R.A. (2006) Advanced Synthesis & Catalysis, 348, 2597–2603. 48 Winkler, M., Glieder, A. and Klempier, N. (2006) Chemical Communications, 12, 1298–1300.

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16 Nitrilases in the Enantioselective Synthesis of α-Hydroxycarboxylic Acids Fred van Rantwijk, Cesar Mateo, Andrzej Chmura, Bruno C. M. Fernandes and Roger A. Sheldon

16.1 Routes to Enantiomerically Pure α-Hydroxycarboxylic Acids

Interest in the synthesis of enantiopure 2-hydroxycarboxylic acids via asymmetric enzymatic transformations is still increasing and two pathways have risen into prominence recently. The first is based on enantioselective hydrocyanation of the appropriate aldehyde in the presence of an oxynitrilase (hydroxynitrile lyase, EC 4.1.2.10), which gives rise to the corresponding enantiomerically pure cyanohydrin, followed by chemical hydrolysis in the presence of strong acid (Figure 16.1, route a). This latter step generates copious quantities of salt and is not compatible with sensitive functional groups, which is a serious limitation. Alternatively, enantiopure 2-hydroxycarboxylic acids can be obtained via a dynamic kinetic resolution of the (chemically synthesized) cyanohydrin in the presence of an enantioselective nitrilase (EC 3.5.5.1) (see Figure 16.1, route b). Racemization of the cyanohydrin, via reversible dehydrocyanation, takes place readily at pH 7 or above. The methodology [1] is attractive on account of the mild reaction conditions and is industrially applied in the multiton-scale synthesis of (R)-mandelic acid [2]. This nitrilase dynamic kinetic resolution (DKR) methodology depends on the availability of highly enantioselective biocatalysts that generate a minimum amount of amide. This latter issue may seem trivial and has long been disregarded somewhat, but reports of modest amounts of amide co-products date back to the early days of nitrilase enzymology. Only recently has the subject come under more intense scrutiny [3–5] and has a relationship with the stereochemistry of the nitrile been demonstrated [3, 5]. Hence, we set out to investigate the enantiomer and chemical selectivity of nitrilases in the hydrolysis of a representative set of cyanohydrins.


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16 Nitrilases in the Enantioselective Synthesis of a-Hydroxycarboxylic Acids

Figure 16.1 Synthetic routes to enantiomerically pure 2hydroxycarboxylic acids, via oxynitrilase (hydroxynitrile lyase) catalysed enantioselective hydrocyanation (route A) and (R)nitrilase (nitrilase) mediated dynamic kinetic resolution (route B). Table 16.1 Nitrilase selectivity in the hydrolysis of 2-hydroxynitriles.a

Cyanohydrin

Nitrilaseb

Initial rate (mmol mg−1 min−1)

eeacid (3, % R)

Amide (4, %)

NIT-104 NIT-106 NIT-107 PfNLase

0.2 0.5 0.014 4.3

98 91 95 40

98% [35]. Chloroperoxidase is the most versatile peroxidase with better stability compared to other peroxidases, because spontaneous oxidation can be suppressed in the presence of ascorbic acid or dihydroxyfumaric acid, and with better enantioselectivity because substrate access to the heme iron and ferryl oxygen favors stereoselective oxygen transfer [36]. Chloroperoxidase has been used for catalyzing the oxidation of cis-cyclopropylmethanols with much higher enantioselectivity than trans-isomers [37]. An interesting oxidative coupling mechanism has been proposed for the biotransformation of p-coumaric acid with peroxidase, leading to dimer and trimer structures with stronger antioxidant activities [38].

20.4 Biocatalytic Asymmetric Oxidations with Dehydrogenases

The broad range of enantioselective dehydrogenases also shows interesting applications in the direction of oxidations. In the production of (R)-1,3-butandiol, the enzymatic resolution by the enantioselective oxidation of the undesired (S)-1,3butanediol in the racemate has been found to be the most practical process [39]. The responsible enzyme was (S)-1,3-butanediol dehydrogenase, a novel secondary alcoholdehydrogenase, from Candida parapsilosis. The use of D-sorbitol dehydrogenase in the oxidation of D-sorbitol to L-sorbose in vitamin C synthesis [9] or the oxidation of N-protected 1-amino-D-sorbitol to the key intermediate in miglitol synthesis [40] are classical examples of the industrial utility of polyol dehydrogenases in the selective oxidation of a single hydroxyl group in complex molecules with various other hydroxyl groups and other oxidizable functional groups. Bypassing a number of the chemical protection–deprotection steps required in conventional non-selective oxidations has been the key success factor for the application of these dehydrogenases in selective single-step oxidations. The selective oxidation of each of the hydroxyl groups of the steroid backbone in cholic acid has been achieved by various hydroxysteroid dehydrogenases [41]. The oxidation of mesodiols with secondary hydroxyl functions to (S)-α-hydroxyketones by glyceroldehydrogenase is an interesting direct route to chiral hydroxyketones [42]. The desymmetrization of meso-diols or prochiral diols by alcohol dehydrogenase has been used as a valuable and selective two-step one-pot method for the synthesis of chiral lactones [43]. A wide variety of prochiral and meso-diols, including sterically demanding bicyclic meso-diols [44–45], have been oxidized by alcohol dehydrogenase to the corresponding chiral lactones with excellent enantiomeric purity. As the chemical resolution of racemic lactols suffers from low enantioselectivity, alcohol dehydrogenases provide the method of choice.

20.5 Biocatalytic Asymmetric Oxidations with Monooxygenases


It is clear that biosynthesis and biodegradation are crucially dependent on selective asymmetric oxidations with easily available natural catalysts and oxidants.Oxygen is available within many environments as an abundant natural oxidant, thereby acting as an electron acceptor. The reactions where one of the atoms of molecular oxygen is introduced into an organic substrate and the other into a molecule of water are catalyzed by monooxygenases. An overview of useful reactions, such as hydroxylations, epoxidations, Baeyer–Villiger oxidations, heteroatom oxidations, and heteroatom dealkylations, is illustrated in Figure 20.5. The oxygen is acting as both an electron acceptor as well as an oxygen transfer agent in these reactions. The repertoire of enzymes for all kinds of asymmetric oxidations is therefore of much interest to the synthetic chemist, because many reaction types, like asymmetric hydroxylations at a saturated carbon atom or oxidative desymmetrizations, are perfectly achieved with the aid of biocatalysts, while being a problem with stoichiometric oxidations and organic or inorganic catalysis. Both chemical and enzymatic synthetic methods for the asymmetric oxidation of the carbon–carbon double bond have been developed [46], but the area of carbon–carbon double bond oxidations has been shaped by the breakthrough discovery of asymmetric epoxidation of allylic alcohols with the Katsuki–Sharpless method [47]. Catalytic asymmetric synthesis of epoxides from alkenes by Jacobsen [48], the catalytic enantioselective epoxidation of olefins and the desymmetrization of meso-tetrahydrofurans by Katsuki [49], and the development of versatile new catalysts [50] have broadened the scope of these reactions. The excellent enantioselectivity of biocatalysts in the epoxidation of terminal olefins to give optically pure R-(+)-epoxides was discovered in 1978 [51] and the selective oxidation of one out of several C−C double bonds in an asymmetric way is a competence of biocatalytic systems [52]. In asymmetric Baeyer–Villiger oxidation and asymmetric heteroatom oxidation, biocatalytic procedures are well ahead of their chemical counterparts [53]. When considering the safety, health, and environment aspects of many types of oxidation reactions [1], the replacement of nitric acid oxidation of carbon feedstocks by more environmentally friendly procedures is highly desirable [54]. The road to the elimination of stoichiometric waste, which can contribute to global warming, ozone depletion, acid rain, and smog, needs catalytic process reengineering and refining and the quest for sustainable chemistry puts the monooxygenases in a central role for a variety of established asymmetric oxidations as well as towards new asymmetric oxidation processes. The cofactors utilized by monooxygenases for the generation of the oxygen-transferring molecules contain either a heteroaromatic molecule like flavin or pterin or a transition metal like iron or copper in a heme or non-heme complex. Heme-dependent monooxygenases contain ferric protoporphyrin IX, the heme, as the cofactor and protect us from the xenobiotics, toxins, food, and chemicals we take up. As these proteins exhibit a very strong absorption band maximum at

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20 Biocatalytic Asymmetric Oxidations with Oxygen

Figure 20.5 Overview of monooxygenase reactions in synthetic chemistry.

450 nm in a carbon monoxide difference spectrum, when carbon monoxide is added, they are also referred to as the cytochrome P450 superfamily. With more than 500 isozymes they are widespread in nature and and catalyze a variety of asymmetric oxidations (see Figure 20.5), which are difficult to achieve by classical organic chemistry. Stereospecific hydroxylations at non-activated carbon atoms under physiological conditions, epoxidations, and heteroatom dealkylations in organisms ranging from bacteria to humans are among the specialty oxidation reactions the heme-dependent monooxygenases are able to catalyze with remark-


able selectivity. The structures of three intermediates in the camphor hydroxylation reaction have revealed insight into the reaction pathway and the mechanism of oxygen activation [55]. An interesting selectivity of cytochrome P450 monoxygenase BM-3 from Bacillus megaterium is the enantioselective epoxidation of the E-isomer geranylacetone to the 9,10-epoxygeranyl-acetone exclusively, while the oxidation of the Z-isomer nerylacetone resulted in a product distribution [56]. This well-studied cytochrome P450 monooxygenase BM-3 has been engineered to perform selective hydroxylation at the 2-position of 2-arylacetic acid derivatives and the 6-position of the anti-anxiety agent buspirone with excellent enantiomeric excess [57]. Non-heme iron-containing monooxygenases are able to catalyze the oxidation of hydrocarbons to the corresponding hydrocarbon alcohols and have been found in methanotrophic bacteria [58]. Dopamine hydroxylase belongs to the copper-dependent monooxygenases and is important in the control of the neurotransmitter concentrations of dopamine and norepinephrine [59]. The important metabolism of the neurotransmitters norepinephrine, epinephrine, dopa, and serotonin involves pterin-dependent monooxygenases. The direct biocatalytic hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophane requires tetrahydrobiopterin and Fe2+ as the cofactors [60]. The cleavage of unsaturated glyceryl ethers by glyceryl ether monooxygenase also requires tetrahydrobioterin as the cofactor [61]. Flavin-dependent monooxygenases are of increasing interest [62] and have been useful in preparative biocatalytic asymmetric Baeyer–Villiger oxidations due to their excellent enantioselectivity [63–65]. Baeyer–Villiger monoxygenases [66] are a group of enzymes that can not only perform asymmetric oxidations of linear and cyclic ketones to their corresponding esters and lactones, but also achieve N- or S-heteroatom oxidations in a highly enantioselective way. As these chiral compounds are difficult to obtain by direct classical oxidations and are useful intermediates in the production of pharmaceuticals, flavors, fragrances, and agrochemicals, the development of robust and scalable processes is a key goal [66]. The production of recombinant Baeyer–Villiger monooxygenases and the identification of reaction and downstream processing bottlenecks has led to the development of large-scale processes [67–69]. The hurdles to further large-scale oxygenase applications have been described to include, among other factors, substrate uptake, substrate toxicity and also oxygen mass transfer [70]. Technical improvements using sinter-metal spargers and the proper balance of cell concentration and oxygen mass transfer [71] have been shown as key success factors for an optimized biocatalytic asymmetric Baeyer–Villiger oxidation process. Symmetric prochiral ketones are of special interest as a biocatalytic Baeyer–Villiger oxidation yields only one chiral product with complete conversion. Resembling this desymmetrization of a prochiral ketone and of similar high synthetic value is the asymmetric oxidation of a thioether to a chiral sulfoxide [72]. Non-symmetric racemic ketones (see Figure 20.6) can be either kinetically resolved by the selective oxidation of only one ketone enantiomer, leaving the opposite enantiomer unchanged and then separating the

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Figure 20.6 Large-scale biocatalytic asymmetric Baeyer–Villiger oxidations.

product lactone from the remaining ketone or by enantiodivergent oxidation of both enantiomers of the racemic ketone and the separation of the resulting regioisomeric lactones. The ability to oxidize racemic bicyclic diketones selectively, such as the Wieland–Miescher and the Hajos–Parrish diketones to the corresponding lactones in a highly regio- and enantioselective manner [73] or the ability to perform a Baeyer-Villiger oxidation of a glycosylated substrate [74], further emphasize the exquisite selectivity of these biocatalysts. The first example of a catalytic enantioselective epoxidation by cyclohexanone monooxygenase was shown with a fosfomycin-related model compound [75]. The efficient asymmetric epoxidation of styrene to (S)-styrene oxide by recombinant styrene monooxygenase has been achieved by increasing biocatalyst concentrations and reducing the exposure time of the biocatalyst to the product [76]. A large range of microbial hydroxylations of aliphatic and aromatic hydrocarbons, terpenes, and steroids is of flavor and fragrance, pharmaceutical, and

20.6 Biocatalytic Asymmetric Oxidations with Dioxygenases

fundamental interest, because the hydroxylation of non-activated carbon centers has only very few counterparts in classical organic chemistry [10, 11, 13, 18, 77].


The most important discovery of the Sharpless asymmetric cis-vicinal dihydroxylation [78] reaction in twentieth-century organic chemistry has its counterpart in nature where the microbial dioxygenation of aromatic compounds became known through the characterization of the metabolic pathway of aromatic compound degradation by Gibson and coworkers [79–82]. Mutant bacterial strains containing dioxygenases but lacking the corresponding diol dehydrogenase activity [80] have been developed for the asymmetric dihydroxylation of a remarkable range of aromatic compounds to cis-dihydrodiols with excellent stereoselectivity [81–83]. These reactions, where both of the atoms of molecular oxygen are introduced into an organic substrate, are catalyzed by dioxygenases and represent highly interesting, safe, and environment-friendly transformations to synthons for organic chemistry [84–86]. The further extension of biocatalytic dihydroxylations (see Figure 20.7) has been facilitated by biodiversity [87], the recombinant protein expression of dioxygenase genes in Escherichia coli [88–90], and directed evolution [91]. Recombinant dioxygenases were applied in the determination of the absolute configuration of metabolites from the toluene dioxygenase-catalyzed dihydroxylation of bromo-(methylsulfanyl)benzenes [92] and from the chlorobenzene dioxygenasecatalyzed asymmetric dihydroxylation of cinammonitrile [93]. Recombinant toluene dioxygenase and chlorobenzene dioxygenase whole cell biocatalysts can perform cis-vicinal dihydroxylation of aromatic nitriles to nitriledihydrodiols [94], the isolated yield of which can be improved by in situ product recovery and which can also be hydrolyzed by nitrilase to the corresponding carboxylic acid [95]. More than 300 cis-vicinal diol-metabolites have now been prepared with dioxygenases [96] from aromatic substrates, a selection of which is shown in Figure 20.8. Recent evidence for the synthesis of tetraoxygenated chiral metabolites [97] by the action of toluene dioxygenase and biphenyl dioxygenase shows that cis-dihydrodiols and acetonide derivatives can themselves be substrates for these enzymes. The class of intramolecular dioxygenases mentioned above incorporates both oxygen atoms into the same molecule and can perform selective oxidations not only on aromatic but also aliphatic substrates. The activation and incorporation of oxygen into the substrates can thereby occur via different mechanisms to highly reactive intermediates like hydroperoxides, endoperoxides, or dioxetanes. Lipoxygenases and cyclooxygenases can incorporate oxygen into an achiral polyunsaturated fatty acid to form a chiral product with high regio- and stereochemical purity [98]. The biocatalytic asymmetric incorporation of oxygen by soybean lipoxygenase into polyunsaturated fatty acids with a non-conjugated 1,4-diene unit like linoleic

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Figure 20.7 Overview dioxygenase reactions in synthetic chemistry.

acid yields chiral hydroperoxides [99]. Cyclooxygenases catalyze the oxidation of arachidonic acid to interesting cyclic endoperoxides and prostaglandins [100]. Dioxygenase-catalyzed oxidative cleavage of carbon–carbon double bonds was discovered by Osamu Hayaishi in his pioneering studies of tryptophan metabolism in 1957 and continues to be of fundamental biochemical interest [101]. Whether the central cleaving enzyme of β-carotene into two molecules of retinal is a monooxygenase [102, 103] or a dioxygenase [104] has been discussed. Isotopic labeling


Figure 20.8 Diversity of aromatic substrates for biocatalytic asymmetric cis-dihydroxylations.

18 experiments using H18 2 O or O2 showed that, in the excentral biocatalytic cleavage, the oxygen in the keto-group of β-ionone is derived solely from molecular dioxygen [104]. In any case, selective biocatalytic cleavage of double bonds in polyenes in a central or excentral manner is an important path to central or remote metabolites.

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The biocatalytic single-step cleavage of arylalkenes is an interesting synthetic tool and an equivalent to reductive ozonization [105]. The intermolecular dioxygenases incorporate the two oxygen atoms into two molecules and the α-keto acid-dependent enzymes are important in the biosynthesis of a variety of biological compounds like penicillins, cephalosporins, and clavulanic acid. Usually the α-keto acid is α-ketoglutarate, which is oxidatively decarboxylated and one of the atoms from molecular oxygen is incorporated into the product succinate. The decarboxylation of α-ketoglutarate can thereby be coupled by these enzymes with the formation of a high-energy ferryl-oxo intermediate acting as a hydrogen-abstracting species for performing hydroxylation or halogenation [106].

20.7 Biocatalytic Asymmetric Oxidations with Other Enzymes

Many unusual biocatalytic asymmetric oxidation reactions like oxidative cyclization, oxidative ring expansion, oxidative deamination, or oxidative decarboxylation were discovered in the course of studies in natural product biosynthesis and the involved enzyme functions continue to be of great interest. In addition to the already mentioned dehydrogenases, biocatalytic oxidative cyclizations have been achieved by a variety of enzyme classes like synthases or tyrosinases as shown in Figure 20.9. Isopenicillin N synthase catalyzes the two iron-dioxygen-mediated oxidative ring closure of the tripeptide δ-(L-αaminoadipoyl)- L-cysteinyl-D-valine to isopenicillin N, the precursor to all penicillins and cephalosporins [107]. The four-electron oxidation of δ-(L-αaminoadipoyl)- L-cysteinyl-D-valine by isopenicillin N synthase is unusual, requiring no 2-oxoglutarate cofactor. In contrast, clavaminate synthase is a remarkable Fe(II)- and 2-oxo-glutarate-dependent oxygenase, which catalyzes the construction of the bicyclic clavam ring system of clavaminic acid from a monocyclic β-lactam. Clavaminate synthase thereby catalyzes the three quite different oxidation reactions of hydroxylation, oxidative cyclization to dihydroclavaminic acid, and the final desaturation reaction to clavaminic acid [108]. Tyrosinase-catalyzed oxidative cyclization of N,N-dimethyltyramine with oxygen has been used for the synthesis of the anachelin chromophore [109]. Selected examples of other biocatalytic asymmetric oxidations are shown in Figure 20.10. In the area of the polyether ionophore monensin a recently proposed mechanism of oxidative cyclization of a linear polyketide intermediate by four enzymes, the products of monBI, monBII, monCI, and monCII, has been supported experimentally by analysis of a biosynthetic gene cluster [110] and the accumulation of an E,E,E-triene, when oxidative cyclization was blocked [111]. Oxidative ring expansion by an additional carbon atom is performed by expandases different from the Baeyer–Villiger monooxygenases capable of introducing oxygen into cyclic ketones. The Fe(II)- and 2-oxoglutarate-dependent oxygenase

20.7 Biocatalytic Asymmetric Oxidations with Other Enzymes

Figure 20.9 Biocatalytic oxidative cyclization reactions.

deacetoxy-cephalosporin C synthase [112] catalyzes the unusual oxidative ring expansion of penicillin N to deacetyoxycephalosporin C. Useful whole cell asymmetric oxidations of methyl groups to carboxyl groups have been found by classical microbiological screening as for the microbial oxidation of prochiral 2-methyl-1,3-propandiol to (R)-3-hydroxy-2-methyl propionic acid with A. pasteurianus with 97% e.e. and 100% molar conversion [113].

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Figure 20.10 Selected examples of other enzymes for asymmetric oxidative transformations.

Acknowledgments

20.8 Outlook

The organic chemistry of enzyme-catalyzed asymmetric oxidations shows a remarkable selectivity and versatility that is best used in an early phase of preceding and subsequent synthesis steps [114]. It will be interesting to establish which complexity of the catalyst is needed for a given asymmetric oxidation, that is whether a multienzyme complex or a multifunctional enzyme, a monofunctional enzyme, or an organocatalyst like an amino acid [115] will be best. As large-scale industrial oxidations of organic compounds with oxidants in flammable solvents pose not only safety, health, and environmental challenges, but often suffer from insufficient selectivity, the development of selective catalysts is of prime importance. Asymmetric Baeyer–Villiger oxidations by biocatalysts have been established on a large scale with a selectivity not reached by the use of inorganic, organometallic, or organocatalysts [116]. The practical process improvements at the macroenvironmental level go in parallel with the improved compatibility of labile functional groups and increased selectivity at the molecular level. Biological cells make use of the required selectivity in many asymmetric oxidation reaction steps of biosynthetic pathways to central and remote metabolites. The elucidation of novel reaction steps in the biosynthesis of known natural products is therefore an interesting entry point for finding the novel biocatalytic reaction methodologies of a wide variety of compounds. An example of such a reaction step is the eightelectron oxidation of chlorinated chromopyrrolic acid to the rebeccamycin aglycone performed by the enzyme pair RebP and RebC [117]. The mode of action has been investigated by crystallography and suggested an interesting interaction: while the first enzyme RebP produces low levels of a reactive intermediate by an eight-electron oxidation, the second enzyme RebC is needed for accelerating rebeccamycine aglycone formation and eliminating side reactions [118]. The combination of molecular biology, chemistry, and process design will be a major driving force for the implementation of biocatalytic asymmetric oxidations [119, 120]. The discovery and understanding of novel biocatalytic oxidations shows great promise for the synthesis of more complex molecules.

Acknowledgments

I would like to thank all co-workers and members of the EU projects on Baeyer– Villiger oxidation, the Swiss project on asymmetric dihydroxylation for their dedicated work and excellent cooperation, and the following organizations for their support: the COST Action D25 and the COST Office, the Swiss State Secretariat for Education and Research, the program SPP-Biotech of the Swiss National Science Foundation, and the EU Research Directorate General.

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Chemie – International Edition, 45, 2501–2503. Blasiak, L.C., Vaillancourt, F.H., Walsh, C.T. and Drennan, C.L. (2006) Crystal structure of the non-haem iron halogenase SyrB2 in syringomycin biosynthesis. Nature, 440, 368–371. Burzlaff, N.I., Rutledge, P.J., Clifton, I.J., Hensgens, C.M., Pickford, M., Adlington, R.M., Roach, P.L. and Baldwin, J.E. (1999) The reaction cycle of isopenicillin N synthase observed by X-ray diffraction. Nature, 401, 721–724. Zhang, Z., Ren, J., Stammers, D.K., Baldwin, J.E., Harlos, K. and Schofield, C.J. (2000) Structural origins of the selectivity of the trifunctional oxygenase clavaminic acid synthase. Nature Structural and Molecular Biology, 7, 127–133. Gademann, K., Bethuel, Y., Locher, H.H. and Hubschwerlen, C. (2007) Biomimetic total synthesis and antimicrobial evaluation of anachelin H. The Journal of Organic Chemistry, 72, 8361–8370. Oliynyk, M., Stark, C.B.W., Bhatt, A., Jones, M.A., Hughes-Thomas, A., Wilkinson, C., Oliynyk, Z., Demydchuk, Y., Staunton, J. and Leadlay, P.F. (2003) Analysis of the biosynthetic gene cluster for the polyether antibiotic monensin in Streptomyces cinnamonensis and evidence for the role of monB and monC genes in oxidative cyclization. Molecular Microbiology, 49, 1179–1190. Bhatt, A., Stark, C.B.W., Harvey, B.M., Gallimore, A.R., Demydchuk, Y.A., Spencer, J.B., Staunton, J. and Leadlay, P.F. (2005) Accumulation of an E,E,Etriene by the monensin-producing polyketide synthase when oxidative cyclization is blocked. Angewandte Chemie – International Edition, 44, 7075–7078. Valegard, K., Terwisscha van Scheltinga, A.C., Lloyd, M.D., Hara, T., Ramaswamy, S., Perrakis, A., Thompson, A., Lee, H.J., Baldwin, J.E., Schofield, C.J., Hajdu, J. and Andersson, I. (1998) Structure of a cephalosporin synthase. Nature, 394, 805–809. Molinari, F., Gandolfi, R., Villa, R., Urban, E. and Kiener, A. (2003) Enantioselective oxidation of prochiral

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21 Second Generation Baeyer–Villiger Biocatalysts Veronique Alphand, Marco W. Fraaije, Marko D. Mihovilovic and Gianluca Ottolina

21.1 Introduction

The oxidative conversion of a ketone into a lactone or ester functionality was first discovered in 1899 by Adolf von Baeyer and Victor Villiger [1]. The transformation turned out to be particularly valuable, as the rearrangement process commences with strict retention of the configuration at the migrating carbon center and the effects governing the regiochemistry of this reaction (conformational, steric, and electronic effects) have been well understood and became highly predictable [2, 3]. In present day organic chemistry, peroxyacids are used as reagents for this oxygen insertion into a less activated carbon-carbon bond and also asymmetric variants have been proposed [4]. The enzyme mediated equivalent was first discovered in 1948 in the biocatalytic degradation of steroids [5] and later proved to be abundant in fungi and prokaryotic organisms. The transformation was established to be dependent on oxygen and a nicotinamide reduction equivalent [6]. Approximately two decades after the biocatalytic Baeyer–Villiger reaction was observed, first Baeyer–Villiger monooxygenases (BVMOs) were isolated and characterized as responsible catalytic entities [7, 8]. In these proteins, a flavin co-factor replaces the peracid of the chemical transformation while NADH or NADPH is required as an electron donor. It is noteworthy that most reported BVMOs are soluble proteins, in contrast to many other types of monooxygenases, which tend to be associated to membranes. Depending on the nature of the flavin co-factor, at least two types of BVMOs can be classified [9]. Type 1 BVMOs consist of a single polypeptide chain, contain flavin adenine dinucleotide (FAD) as a tightly bound co-factor, and are NADPH dependent: the two co-factors are bound at separate dinucleotide binding domains as indicated by the presence of two Rossmann sequence motifs (GxGxxG). Type 2 BVMOs contain flavin mononucleotide (FMN) and are dependent on NADH: they are composed of two different subunits and display some relationship with flavin-dependent luciferases. The generally accepted mechanism of BVMOs has been exemplified using a BVMO from Acinetobacter [10, 11]. Initially, the NADPH-mediated reduction of the FAD moiety leads to formation of an enamine-type intermediate (Figure 21.1). Modern Biocatalysis: Stereoselective and Environmentally Friendly Reactions. Edited by Wolf-Dieter Fessner and Thorleif Anthonsen Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32071-4

Figure 21.1 Mechanism and catalytic cycle of flavin-containing BVMOs.

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21.2 BVMO Enzyme Platform

This species reacts readily with molecular oxygen to yield the 4a-peroxyanion as equivalent to an organic peracid. Nucleophlic attack of the anion at a carbonyl group leads to formation of the Criegee intermediate analogous to the conventional reaction mechanism. Now, both stereoelectronic and conformational effects govern the rearrangement process: the spatial and electrostatic composition of the enzyme’s active site determine which C−C bond can adopt the antiperiplanar conformation vis-à-vis the leaving group as a key aspect for migratory preference. The ester or lactone formed are released from the active site and the catalytic cycle is closed by elimination of water to regenerate the flavin co-factor. The peroxyanion is also in equilibrium with the corresponding hydroperoxide and this protonation is believed to become prominent in the absence of a target for the nucleophilic attack. Consequently, an electrophilic reactive species is formed, which is then responsible for the oxygenation processes of heteroatoms. Substantial progress in the understanding of this enzyme family was achieved by the recent determination of the three-dimensional structure of phenylacetone monooxygenase (PAMO) from the moderately thermophilic bacterium Thermobifida fusca [12], which exhibits a substrate preference for aryl containing ketones [13]. The enzyme displays a two-domain architecture: the FAD-binding domain is linked to the NADPH-binding domain and the active site is located in a cleft at the domain interface. The linker region contains a highly conserved sequence motif which represents an indicative ‘fingerprint’ sequence (FxGxxxHxxxW) for all BVMOs [14]. The two-domain set-up of PAMO suggests substantial conformational variations in the course of the biocatalytic oxygenation process. BVMOs display a remarkable substrate promiscuity combined with usually high chemo-, regio-, and stereoselectivity, which make them appealing enzymes for applications in asymmetric synthetic chemistry. In recent years, a substantial number of newly identified enzymes have become available with complementary biocatalytic properties, in particular with respect to chemo- as well as stereoselectivity. While several detailed review articles have been published lately covering aspects of biochemistry [15, 16] and synthetic applications of BVMOs [17, 18] in a historical context, this chapter represents a highlight-type contribution focusing on recent developments in the field in order to promote the application of this enzyme group in organic synthesis further. The main emphasis will be put on modernized approaches for exploiting the well-established knowledge base in BVMO biocatalysis towards a ‘second generation’ of BVMO-type catalytic systems. The aspects discussed will mainly focus on progress resulting from the transnational cooperation by the research groups of the authors within a CERC3 research program in cooperation with the COST Action D25 – Applied Biocatalysis.

21.2 BVMO Enzyme Platform

The majority of applications of enzyme-mediated Baeyer–Villiger oxidations have focused on the conversion of cyclic ketones to chiral lactones because of the impor-

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tance of these building blocks in asymmetric synthesis. Many of the particular features of BVMOs were discovered with the cyclohexanone monooxygenase from Acinetobacter NCIMB 9871 (CHMOAcineto), establishing this enzyme as the most prominent role model [19]. However, a large diversity of different BVMOs have become available within recent years. Various techniques in molecular biology were used for accessing novel genes, either by knowledge-based [14] or random strategies [20]. The applicability of these enzymes was largely improved by the generation of recombinant whole cells (Saccharomyces cerevisiae [21] and Escherichia coli [22] hosts), thereby providing simple solutions to the obstacles associated with protein isolation and co-factor regeneration applicable to synthetic chemists without special know-how in biochemistry. Table 21.1 provides an overview of the most prominent enzymes together with information on representative substrate profiles in a historical context. The presently available BVMO platform includes enzymes that are highly stereoselective for the conversion of cyclic ketones in both desymmetrization reactions and kinetic resolutions capable of converting larger rings or sterically more demanding systems. Valuable extensions are BVMOs capable of selectively oxygenating arylcontaining substrates as well as linear ketones. Based on a phylogenetic relationship, a certain clustering of those enzymes with a relevant number of successfully converted substrates can be observed based on their chemo- and stereoselectivity as outlined in Figure 21.2. Of particular relevance was the recent identification of two groups of cycloketone-converting BVMOs enabling access to enantiocomplementary lactones on a large number of substrates [32, 46, 47]. The enzymes included in this phylogenetic analysis represent only a very minor fraction of tentatively assigned BVMOs from recent genome sequencing programs. Well-defined substrate profiles, including structurally and electronically diverse ketone precursors, are available for many of these biocatalysts. Together with enzyme models derived from the structure of PAMO this may even allow careful predictions for novel substrates and/or modifications of the catalytic performance. However, a large variety of yet uncharacterized enzymes still awaits discovery for novel types of biotransformations.

21.3 Engineering of BVMOs

Altering the enzymatic properties of previously characterized enzymes by mutagenesis methodology using DNA technology is a well-accepted and often-used approach in the field of biocatalysis. By this approach, not only can substrate specificities be optimized, but the stability towards external factors such as temperature, organic solvents, salinity, and pH can also be improved. In recent years it has become popular to employ random mutagenesis combined with functionality screening (‘directed evolution’) for enzyme redesign. However, such an approach is often not very efficient for changing the substrate specificity of a certain biocatalyst as random mutations often occur far from the active site [48].

21.3 Engineering of BVMOs Table 21.1 BVMOs available in recombinant form with demonstrated potential for synthetic applications.

BVMO/Origin

Year

Representative

Identical

Cloned

substrate profile

Baeyer–Villiger monooxygenase (BVMOMtb5) Mycobacterium tuberculosis H37Rv

2006 [23]

2006 [23]

[23, 24]

Cyclododecanone monooxygenase (CDMO) Rhodococcus SC1

2001 [25]

2001 [25]

[26]

Cyclohexanone monooxygenase (CHMOAcineto) Acinetobacer NCIMB 9871

1976 [27]

1988 [28]

[29]

Cyclohexanone monooxygenase (CHMOArthro) Arthrobacter BP2

2003 [30]

2003 [31]

[26, 32]

Cyclohexanone monooxygenase (CHMOBrachy) Brachymonas petroleovorans

2003 [33]

2003 [33]

[26, 32]

Cyclohexanone monooxygenase (CHMOBrevi1&2) Brevibacterium HCU

2000 [34]

2000 [34]

[26, 35]

Cyclohexanone monooxygenase (CHMORhodo1&2) Rhodococcus Phi1 and Phi2

2003 [30]

2003 [30]

[26, 32]

Cyclopentadecanone monooxygenase (CPDMO) Pseudomonas HI-70

2006 [36]

2006 [36]

[36]

Cyclopentanone monooxygenase (CPMOComa) Comamonas NCIMB 9872

1976 [37]

2002 [38]

[32, 38]

Hydroxyacetophenone monooxygenase (HAPMO) Pseudomonas fluorescens ACB

2001 [39]

2001 [39]

[40, 41]

Linear ketone monooxygenase (BVMOPflu) Pseudomonas fluorescens DSM 50106

2006 [42]

2006 [42]

[42]

Phenylacetone monooxygenase (PAMO) Thermobifica fusca

2004 [43]

2005 [44]

[45]

Such long-distance mutations typically do not strongly influence the substrate acceptance profile of enzymes. In fact, it appears that, for effectively altering the substrate specificity of an enzyme, one should preferentially target active site residues. Such a targeted mutagenesis of ‘first shell’ residues has been shown to result in dramatic changes in substrate specificity and/or enantioselectivity [49]. However,

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Figure 21.2 Phylogenetic relationships within BVMOs. (sequences of 18 proteins with confirmed BVMO activity were aligned and an unrooted phylogenetic tree was calculated using Clustal X v. 1.83 and TreeView v. 1.4).1

1) Abbreviations and GenBank accession numbers of protein sequences: CHMO Acinetobacter: CHMO Acinetobacter sp. NCIMB 9871: BAA86293; CHMO2 Brevibacterium: CHMO 2 Brevibacterium sp. HCU: AAG01290; CHMO Arthrobacter: CHMO Arthrobacter sp. BP2: AAN37479; CHMO Brachymonas: CHMO Brachymonas petroleovorans: AAR99068; CHMO1 Brevibacterium: CHMO 1 Brevibacterium sp. HCU: AAG01289; CHMO1 Rhodococcus: CHMO Rhodococcus sp. Phi1: AAN37494; CHMO2 Rhodococcus: CHMO Rhodococcus sp. Phi2: AAN37491; CHMO Xanthobacter: BVMO Xanthobacter sp. ZL5: CAD10801; CPMO Comamonas: cyclopentanone monooxygenase Comamonas sp. NCIMB 9872: BAC22652; PAMO T. fusca:

phenylacetone monooxygenase Thermobifida fusca: 1W4X_A; HAPMO P. fluorescens: 4hydroxyacetophenone monooxygenase Pseudomonas fluorescens: AAK54073; STMO R. rhodochrous: steroid monooxygenase Rhodococcus rhodochrous: BAA24454; CDMO R. rubber: cyclododecanone monooxygenase Rhodococcus rubber: AAL14233; Rv3854c M. tuberculosis: ETaA Mycobacterium tuberculosis H37Rv: CAB06212; CPDMO Pseudomonas: cyclopentadecanone monooxygenase Pseudomonas sp. HI-70: BAE93346; Rv3049c M. tuberculosis: BVMO from Mycobacterium tuberculosis H37Rv: CAA16134; Rv3083 M. tuberculosis: BVMO from Mycobacterium tuberculosis H37Rv: CAA16141; BVMO P. fluorescens: BVMO from Pseudomonas fluorescens: AAC36351.

21.3 Engineering of BVMOs

Figure 21.3 Overlay of the active site of PAMO (dark gray) and the model of CPMO (light gray). The FAD co-factor is in black. The three non-conserved residues that have been mutated in PAMO are indicated by the respective numbers for PAMO and CPMO.

such a structure-inspired (random) mutagenesis approach is only feasible when a structure or a structural model of the targeted biocatalyst is available. In our recent work on BVMOs, we exploited the crystal structure of PAMO, which represents the only BVMO structure presently available [12, 13]. In a first study to alter substrate specificity, the PAMO structure and a CHMO model were compared in order to identify a bulge region in PAMO which appeared to be crucial for substrate specificity. Removing this bulge by deleting a few residues resulted in mutant PAMO enzymes that displayed activity with relatively bulky substrates, which are poorly or not accepted by wild-type PAMO [50]. A comparison of the PAMO structure and a homology-built structural model of the sequencerelated CPMO revealed that both active sites are remarkably similar (Figure 21.3). In fact, the above-mentioned bulge region is also absent in CPMO, while it shows a similar substrate acceptance profile when compared with CHMO. Still, PAMO and CPMO show huge differences in their substrate profiles: while PAMO prefers aromatic substrates, CPMO is active with a wide range of (cyclic) aliphatic ketones. Most residues that surround the flavin co-factor in both BVMOs, thereby forming the active site, were found to be identical. Only three residues could be identified as significantly different: Q152, L153, and M446. It was postulated that these ‘first shell’ residues determine the molecular basis for the two widely different substrate specificities of PAMO and CPMO to some extent. We replaced these residues in PAMO by the corresponding residues found in CPMO, thereby yielding three PAMO mutants [51]. Surprisingly, two of these mutants were found to be inactive (Q152F/L153G and Q152F/L153G/M446G).

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These two mutant proteins were still able to bind the NADPH and FAD co-factors. Nevertheless, the mutants had lost the ability to perform oxygenation reactions. Apparently, the simultaneous mutation of Q152 and L153 resulted in an altered active site in which the organic substrates are not able to be oxidized by the peroxyflavin. In a recent mutagenesis study of CPMO these two homologous residues were randomly modified yielding several mutants with improved enantioselective behavior [51]. However, without exception, all reported CPMO mutants were less active and/or stable when compared with wild-type enzyme as evidenced by poor to moderate conversions using cells expressing these mutant enzymes. Cells containing wild-type CPMO resulted in full conversion of most tested substrates. This indicates that there is a subtle interplay between ‘first shell’ and ‘second shell’ residues resulting in a balanced and suitable substrate binding pocket. Interestingly, the single mutant M446G PAMO was found to display a drastically altered substrate specificity and enantioselectivity. Several new compounds were identified as substrates for this PAMO variant (e.g. indole and benzaldehyde). The mutant displayed a higher specificity towards substrates containing the carbonyl group or the heteroatom in close proximity to the aromatic ring. A different positioning of the substrate’s aromatic ring in the active site might cause this observed shift in regioselectivity. Furthermore, an altered substrate-binding pocket also explains the substantial changes in enantioselectivity observed towards sulfides and ketones. This confirms the role of this specific amino acid residue in modulating the substrate-binding pocket. Interestingly, the active site of mutant M446G is to some extent mimicking the effect of addition of methanol to wild-type PAMO. Recently we have shown that usage of up to 30% methanol is tolerated by PAMO and also affects the enantioselectivity [52]. Whether the organic solvent specifically binds in the active site or induces structural changes in wild-type PAMO, ultimately resulting in an altered substrate-binding pocket, is still disputable. The fact that replacement of M446 can result in dramatic changes in substrate specificity and enantioselectivity suggests that specific interaction of one or two methanol molecules in the active site may explain the observed solvent effect. While the exact binding mode of substrates in PAMO and other BVMOs is uncertain due to the lack of a crystal structure containing a bound substrate or product, the mutagenesis studies described deliver clues about the residues involved in forming the substrate-binding pocket. In the last few years several random and directed mutagenesis studies have been performed with PAMO, CHMO and CPMO [53–56]. An inventory of all mutants that display an alteration of enantioselectivity shows that many targeted residues are part of the first shell of the proposed substrate-binding pocket of PAMO. As can be seen in Figure 21.4, most of these residues align a pocket at the si-face of the flavin co-factor. It is also striking to note that most of the observed mutations are located in the FAD binding domain and the neighboring helical subdomain, while no mutation occurs in the NADPH-binding domain. The identification of M446 in PAMO as being crucially involved in determining the substrate specificity and enantioselectivity complements the previously identified hotspots that influence BVMO specificity (Figure 21.4, Table 21.2).

21.4 Baeyer–Villiger Biooxidation in Synthetic Chemistry

Figure 21.4 Cartoon picture of the PAMO crystal structure. The FAD co-factor is shown in sticks in the center together with the active site R337. Residues that are 99% e.e. (−) 97% e.e. (−) 97% e.e. (−) 99% e.e. (+) 91% e.e. (+)

5% e.e. (−) >99% e.e. (−) 71% e.e. (−) 94% e.e. (+) >99% e.e. (+)

97% e.e. (−) 96% e.e. (−) 94% e.e. (−) 92% e.e. (+) 91% e.e. (+)

CHMOBrevi1 adopts a borderline position CHMOBrevi1 (n.c.: no conversion; the sign of specific rotation is indicated after the value for the e.e.).

mentary lactones are obtained, in some cases one group only exhibits high stereoselectivity, and in the minority of cases a difference in substrate acceptance is observed. The CHMO from Brevibacterium adopts a borderline position between these groups with some similarities to CHMO-type enzymes, albeit displaying a distinct stereospecificity with several substrates (Table 21.3; see also Figure 21.2 for phylogenetic analysis). This suite of cycloketone-converting BVMOs has recently been expanded for enzymes displaying appealing properties in the kinetic resolution of linear ketones. A BVMO from Pseudomonas fluorescens turned out to be particularly useful for the conversion of terminal ketones to chiral diol derivatives [42, 69], while PAMO and HAPMO are suitable biocatalysts for the kinetic resolution of aromatic ketones and aldehydes [70]. The formation of BVMO clusters is also confirmed to a certain extent in regiodivergent biooxygenations by this enzyme group. Migratory preference of the more nucleophilic (higher substituted) center is observed generally for chemical oxidations and in the majority of enzyme-mediated transformations. This behavior is governed by the difference in electron density between carbons bearing an additional group and those with all hydrogen substituents. However, stereoelectronic effects leading to a preferred conformational arrangement to promote a different direction of the rearrangement process is particularly frequently encountered in biotransformations. In selected cases, racemic precursors are then oxidized by BVMOs into two types of regioisomeric lactones in a resolution process: migration of the more substituted carbon atom leads to formation of the expected ‘normal’ lactone, while the ‘abnormal’ lactone is produced upon migration of the less substituted carbon atom (Scheme 21.5).


Scheme 21.5 BVMO portfolio for regiodivergent biooxidation of fused bicycloketones.

While this remarkable behavior was initially observed using CHMOAcineto with fused bicycloketones bearing a cyclobutanone structural motif [71] (see [72] for mechanistic considerations), a more general trend could be identified for CHMOand CPMO-type enzymes: the prior cluster usually displays a clear regiodivergent biooxygenation and racemic precursors are converted into isomeric ‘normal’ and ‘abnormal’ lactones in approximately 1 : 1 ratios and in high optical purities, while representatives of the CPMO group preferentially yield ‘normal’ lactones, albeit in racemic form [73, 74]. The latter catalysts therefore enable chemoselective entry to chiral lactones when starting from optically pure ketone precursors. Recently, screening of a small BVMO library originating from Mycobacterium tuberculosis led to the discovery of BVMOMtb5 as the selective biocatalyst for the predominant preparation of ‘abnormal’ lactones. This enzyme performs a kinetic resolution of racemic precursors to yield enantiomerically pure ketone and lactone, subsequently simplifying the separation of these products by chromatography. In addition, it nicely complements the portfolio of divergent biooxygenations with fused bicyclic ketones incorporating a cyclobutanone structural motif [75, 76]. In the case of larger ring systems, we could most recently apply the available collection of cycloketone-converting BVMOs to access all possible regio- and stereoisomeric lactones from terpenone precursors, as exemplified in Scheme 21.6 for biotransformations of dihydrocarvone [77]. Isolated cases, in which the formation of an unexpected regioisomer was observed, included CHMOAcineto-mediated oxygenation of cyclohexanones bearing an electron-withdrawing cyano substituent in the 2-position [78] as well as the biotransformation of β-substituted cycloketones [79, 80]. Certainly, there remains ample opportunities for contributing to this

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Scheme 21.6 Regiodivergent biooxidation of dihydrocarvone.

interesting sub-domain, in which enzymatic transformations enable rapid entry to diverse chiral building blocks that are otherwise difficult to obtain via conventional organic chemistry. 21.4.4 Natural Product and Bioactive Compound Synthesis

All of the strategies in the Baeyer–Villiger biooxygenations outlined above enable efficient access to optically pure building blocks of relevance for the synthesis of bioactive compounds or natural products. Several appealing applications have already been outlined in the past. Both chiral lactones or ketones obtained from kinetic resolutions of simple cycloketones have been utilized for the production of lipoic acid [81, 82] and various pheromones [83]. In addition, both products have been applied in synthetic routes to prostaglandins [84], algae pheromones [85], and cytostatics [86] in regiodivergent biooxygenations. More recent applications in target-oriented synthesis took advantage of the BVMO platform for the generation of enantiocomplementary lactones. In this context, butyrolactones represent appealing intermediates due to the facile availability of prochiral ketone substrates for enzyme-mediated desymmetrizations; Scheme 21.7 indicates the potential in lignan total synthesis based on products obtained in a previous study [46]. The collection of BVMOs exploited by our groups also allowed efficient access to various indole alkaloids via enantiocomplementary lactones obtained in the desymmetrization of fused bicycloketones [35].


Scheme 21.7 Microbial Baeyer–Villiger oxidation of prochiral cyclobutanones to access antipodal butyrolactones as a platform for the synthesis of various lignans.

Bridged bicycloketones are attractive substrates due to the structural complexity of the scaffolds. The high chemoselectivity of the biotransformation for the carbonyl oxygenation allows for a functional decoration without the need for elaborate protection strategies. Subsequent chemical elaboration of the biooxidation products opens up rapid and efficient novel synthetic pathways towards various target compounds. Biotransformation of hetero-bicyclic ketone 8 enabled access to unsaturated lactone 9 in a desymmetrization reaction without compromising the olefin func-

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Scheme 21.8 BVMO-mediated desymmetrization of prochiral hetero-bicycloketone 8 and subsequent synthetic elaboration towards structurally diverse natural products.

tionality. This intermediate was then converted to showdomycin as a structural representative of the compound class of C-nucleoside antibiotics in order to establish the absolute configuration of this novel BVMO-derived metabolite. Alternative synthetic exploitation of the residual functionalities (alkene and lactone) opened up access to tetrahydrofuran natural products like kumausyne as well as goniofufurone analogs (Scheme 21.8) [87]. Green chemistry methods such as biocatalysis become particularly attractive when combined with additional sustainable strategies: very recently we proposed a combined photochemical and biocatalytic approach to access bicyclo[4.2.0] octanes via a novel strategy. Usually, the Cu-catalyzed [2 + 2] photocycloaddition of distantly located terminal diolefins is very difficult. However, when the double bonds are in close proximity the cyclization reaction proceeds smoothly (11). Consequently, incorporation of a cleavable bridge was expected to open a route to the above target compounds. In a first approach, a keto functionality was established at the bridge (12), which subsequently allowed for several cleavage protocols to be applied. The microbial Baeyer–Villiger oxidation again turned out to be the most appealing approach, as biotransformations proceeded in high yields to 13. In addition, the biocatalytic route allowed access to antipodal lactones with full control over four stereogenic centers utilizing representatives of the CHMO and CPMO groups. Ultimately, the target bicyclo[4.2.0]octane system (14) was obtained after chemical lactone hydrolysis (Scheme 21.9) [88, 89].

21.5 BVMOs in Stereoselective Sulfoxidations

Scheme 21.9 Photochemical Cu-assisted [2 + 2] cycloaddition followed by BVMO-mediated desymmetrization as key steps in a novel synthetic approach to optically active bicyclo[4.2.0]octanes.

21.5 BVMOs in Stereoselective Sulfoxidations

The typical S-oxidation with BVMOs allows the formation of chiral sulfoxides from organic sulfides. This oxidation has received much interest in organic chemistry due to its use in the synthesis of enantiomerically enriched materials as chiral auxiliaries or directly as biologically active ingredients. This reaction has been studied extensively with CHMO from Acinetobacter showing high enantioselectivities in the sulfoxidation of alkyl aryl sulfides, disulfides, dialkyl sulfides, and cyclic and acyclic 1,3-dithioacetals [90]. CHMO also catalyzes the enantioselective oxidation of organic cyclic sulfites to sulfates [91]. Recently the sulfoxidation of some organic sulfides has been tested with two other enzymes: PAMO and HAPMO [92, 93]. The oxidation of sulfides by PAMO or HAPMO was coupled to a second enzymatic reaction to regenerate NADPH using glucose 6-phosphate dehydrogense (G6PDH). With regard to the stereoselectivity of enzymatic reactions, the data indicated that, for PAMO, it is highly dependent on substrate structure. Thus, the optical purities of the products range from 80% e.e. and (R)-configuration with methyl phenethyl sulfoxide to 98% e.e. and the (S)-configuration with benzyl ethyl sulfoxide. The alkyl benzyl sulfides possessing a small alkyl chain appeared to be very good PAMO substrates in terms of selectivity. It is quite interesting to point out that the best PAMO substrate is phenylacetone, which shares a similar structure to alkyl benzyl sulfides, suggesting that this structural motif is the preferred by this enzyme. Moreover, the e.e. of the sulfoxide products changes with the progress of the reaction indicating that the PAMO was able to catalyze the kinetic resolution of the sulfoxide to the sulfone. The enantiomeric ratio measured with methyl phenethyl sulfoxide was 110. Previous studies carried out with CHMO revealed

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that the oxidation of the sulfoxide to sulfone was very slow, thereby preventing exploitation of this kinetic resolution [90]. With HAPMO the stereoselectivity was generally high: thioanisole oxidation was almost complete after 1 day, resulting in the formation of enantiopure (S)-methyl phenyl sulfoxide. A very high e.e. was also obtained in the sulfoxidation of thioanisole with CHMO (>99%), but with (R)-configuration. This finding was true for several aryl alkyl sulfides suggesting that these two enzymes might be enantiocomplementary in sulfoxidation reactions. PAMO and HAPMO have been used in aqueous organic media in the sulfoxidation of several substrates. The effect of the solvent on the catalytic properties of the enzymes was quite remarkable, apart from a general decrease of enzyme activity. A high increase in enantioselelctivity was found with methanol and PAMO, which was one of the best solvents for this enzyme. However, for HAPMO the best solvent found was i-Pr2O. A pH dependence of enantioselectivity has been observed with thioanisole and PAMO [94], showing that the e.e. increased from 10 to 45% by raising the pH from 6 to 10.

21.6 Towards a Technology Platform 21.6.1 Fermentation Up-Scaling

In addition to the high chemo- and stereoselectivity of BVMOs, the operational benefits of enzymatic oxygenation processes also include safety issues compared to chemical Baeyer–Villiger oxidations, since classical oxidants and flammable solvents are replaced, respectively, by air and water. However, contrary to expectation, no real breakthroughs have been reached at the industrial level. Even at the laboratory-scale, Baeyer–Villiger biocatalysts are under employed by synthetic chemistry [95]. Several explanations can be pointed out: the apparent complexity of the process [96] (e.g. compared to hydrolytic enzymes) with the need of co-factor recycling, but also the importance of providing a high level of oxygenation. Another bottleneck is the low productivity, which arises from a low working concentration in substrate and product due to frequent inhibition or toxicity phenomena. Nevertheless, various techniques were recently developed successfully in order to overcome, or at least minimize, these limitations. 21.6.1.1 Whole Cells One of the easiest means of solving the co-factor need is the use of a whole cellbased process. NAD(P)H recycling is provided by the endogenous cellular tools themselves. The implementation of this process was largely facilitated by the use of recombinant cells overexpressing the target enzyme. The criteria for the host strain selection are simplicity of culture, fast growth rate, absence of natural side reactions, ease of genetic modification, and, of course, existence of an internal

21.6 Towards a Technology Platform

efficient co-factor regeneration system They were mostly fulfilled by E. coli species like E. coli TOP10 [97], E. coli BL21(DE3) [22], and E. coli DH5α [98]. Enzyme overexpression in such systems is under the control of a strong promoter and can be well controlled by induction using arabinose and isopropylthio galacto pyranoside (IPTG). Reducing equivalents, which are crucial for driving bioconversion to completion, are supplied from glucose [99] or glycerol [100] addition. However, whatever the strain, productivity is still rather low with respect to industrial demand. The standard bioconversion of bicyclo[3.2.0]hept-2-en-6-one barely reached a space–time yield of 0.87 g/l/h for an initial substrate concentration of 1 g/l [95c]. Although the parameters governing a whole cell-based process are numerous, the key limitation of the low productivity was clearly identified as substrate and product inhibition. Substrate inhibition could conveniently be overcome by continuous feeding. A pilot-scale fed-batch experiment (55 l) was carried out [101] in which the feeding rate was adjusted to maintain the ketone concentration below 0.7 g/l. A final concentration of about 4 g/l of lactones was reached within 4 h. After downstream product recovery on charcoal, over 200 g of combined lactones was obtained, corresponding to a 76% yield. To improve productivity further, limitation by product inhibition has also to be circumvented, but that implies a deeper modification of the process. A classical method is the use of a two-liquid phase process. Only small-scale experiments were carried out with moderate success [100, 102, 103]. Moreover, given the strong oxygen requirements of the biotransformation (cf. below), the up-scaling of this method would probably be hampered. A quantitative jump was finally brought by a ‘two-in-one’ resin-based in situ substrate feeding product removal method (SFPR) [104]. An adsorbent polymeric resin acts simultaneously as a reservoir for the substrate and as a trap for the product (Figure 21.5). The substrate, preloaded onto the resin, slowly diffuses into the broth and is transformed by cells while the formed product is readsorbed onto the solid. The system is conveniently tuned by a simple choice of substrate : resin ratio. Thus, both the substrate and product concentrations are controlled in order to maintain them below their inhibitory or toxic levels and allow a higher productivity. Additional benefits are also to be considered for practical large-scale applica-

Figure 21.5 Principle of ‘two-in-one’ resin-based in situ SFPR.

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Figure 21.6 Flowsheet of the resin-based SFPR process.

tions such as the protection of sensitive compounds against evaporation or degradation, the simplified extraction protocol, and the reusability of the adsorbent resin. Product can be recovered from resin either by washings with a polar solvent or, to minimize the solvent amount, by a soxhlet extraction. The resin can also be reused after washing with methanol and water. Several engineering designs (recycle reactor, conventional fermentor, and bubble column) bound for testing the SFPR concept at the laboratory scale were described [105]. At best, 25 g/l of bicyclo[3.2.0]hept-2-en-6-one were transformed by whole cells of CHMO-overexpressing E. coli TOP10[pQR239] in a space–time yield up to 1.2 g/l/h. Interestingly, exhausted cells could be replaced by fresh ones [106] by simple resin decantation when needed, which allowed the reaction to go to completion via a second cycle (Figure 21.6). Oxygenation also proved to be crucial [107]: At high cell density, the oxygen requirements of Baeyer–Villiger oxidation compete with the needs of cell metabolism (respiration, co-factor recycling, etc.). The optimum biocatalyst concentration has to be set with respect to the oxygenation ability of a given reactor. Techniques for enhancing oxygen mass transfer such as a longer bubble residence time, the formation of smaller air bubbles (to increase the interface area) [107a], or the use of oxygen-enriched air have been tested [107b]. In the context of industrial applications, where the versatility of the equipment can be decisive for the choice of the process type, the best results were obtained by simply replacing the common fermenter ring sparger by a sintered-metal sparger. In collaboration with Sigma Aldrich, this simple modification was applied to a 50-l conventional industrial fermenter and enabled a 1-kg-scale operation in batch mode in less than 20 h [108]. An amount of 6 kg of adsorbent Dowex Optipore L-493 was used and regioisomeric lactones were directly separated by simulated moving bed chromatography after extraction [95b]. The resin-based SFPR concept is appearing more and more as a general tool for highly regio-, stereo-, or enantioselective Baeyer–Villiger biotransformations. Its scope has been successfully extended to various ketones [98] or sulfides [109] and different expression systems. Thus, typically 1 l of CPMO-overexpressing E.coli DH5α cells in the presence of Lewatit VPOC1163 transformed 5–15 g of

21.6 Towards a Technology Platform

ketones (3- and 4-methylcyclohexanone) into lactones in an approximately 80–95% yield. 21.6.1.2 Enzyme To date, only a few examples of laboratory preparative-scale processes based on purified enzyme have been reported. Several studies have focused on the smallscale implementation of processes associating a new co-factor regenerating system, enzyme immobilization, membrane reactor, continuous substrate feeding, or resin-based SFPR with various results [110]. Using the outstanding stability of PAMO, a 200 ml biotransformation of 5 g/l phenyl cyclohexanone by an engineered mutant under two-liquid phase conditions using methyl tert-butyl ether as solvent was described [102]. 21.6.2 Immobilization of BVMOs

CHMOAcineto, the most used BVMO so far, is inherently unstable: therefore the operational stability is of high importance for any potential bioprocess. Certainly, a way of increasing the operational stability and also facilitating the work-up of the products is the immobilization of the biocatalysts, but there are only a few examples of immobilization of BVMOs. CHMO has been co-immobilized with alcohol dehydrogenase (ADH) or G6PDH as a recycling system for the regeneration of the co-factor (NADPH). CHMO and G6PDH have been co-immobilized with poly(acrylamide-co-N-acryloxysuccinimide) [111] and used for Baeyer–Villiger reactions lasting up to 10 days. More recently CHMO was co-immobilized with ADH on Eupergit (oxirane acrylic beads) [112], showing that the rate of substrate transformation was approximately 80% of that obtained with free enzyme and that the catalyst could be reused for up to 16 days with complete substrate conversion. The half-life of the immobilized CHMO was 2.5 days, which doubled the stability of the free enzyme. CHMO was also immobilized with a polyethylenimine-porous agarose polymeric composite and subjecting to a dose of γ-radiation [113]. After optimization 87% of the activity wais retained and this catalyst was reused for 16 cycles in the Baeyer–Villiger oxidation of a cyclohexanone derivative. 21.6.3 Self-sufficient Fusion Protein BVMOs

Several strategies of co-factor regeneration as one of the key obstacles in redox biocatalyst application have already been outlined in the previous section of this chapter. Most recently, we employed a novel approach to combine the catalytic activity of redox biocatalysts with concomitant co-enzyme recycling in a single fusion protein. During the last decade, a number of fusion protein tags have been developed that are used intensely in life science-related research and commercial activities. While the fusing of proteins is a widely applied strategy in, for example,

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enzyme purification protocols (e.g. GST-tag) [114] and subcellular visualization of target proteins (e.g. GFP-tag), [115] this concept is hardly encountered in the context of synthetic applications. Only a few isolated examples in the literature provide evidence that fusing separate enzymes can result in improved biocatalytic properties [116]. In this particular case, we decided to take advantage of the favorable thermodynamic equilibrium constant that drives the oxidation of phosphite to phosphate mediated by a recently described phosphite dehydrogenase (PTDH) [117] to a nearly irreversible process [118]. The exquisite selectivity of PTDH for phosphite also precludes any side reaction that can occur in case, for example, an ADH is used. These characteristics render PTDH as an ideal candidate for use as a coenzyme regenerating enzyme (CRE) in combination with BVMOs or other NAD(P)H-dependent enzymes. We created covalently connected fusion enzyme biocatalysts between PAMO, CHMOAcineto, or CPMOComa with PTDH (Figure 21.7) (CRE-PAMO, CRE-CHMO, and CRE-CPMO) within a proof-of-concept study [119]. Remarkably, we did not observe a significant change in the kinetic parameters of the bifunctional enzymes. Moreover, the substrate specificity as well as the stereoselectivity of the fusion BVMO–PTDH biocatalysts was hardly affected at all. Consequently, the fusion enzymes displayed a very similar biocatalytic behavior as compared to wild-type proteins. Based on the substrate acceptance profile of the three BVMOs, a first library of self-sufficient biocatalysts is now available for the conversion of aromatic and cycloaliphatic ketones for accessing even enantiocomplementary lactone products. The novel biocatalysts were successfully employed in whole cell and purified protein biotransformations. However, a most appealing mode of operation is the utilization of a crude cell extract, which usually contains a very high amount of the CRE-BVMOs together with sufficient quantities of NADPH from the living host organism, ultimately making the addition of any co-factor recycling agent (additional enzyme, nicotinamide co-factor) obsolete apart from the auxiliary and cheap substrate phosphite. At present, studies to extend this strategy to other enzymes are being conducted in our groups and we hope that this concept may become an appealing solution to the problem of co-factor regeneration in redox biocatalysis.

Figure 21.7 Schematic representation of co-enzyme regeneration by CRE BVMO fusion enzymes.

References

21.7 Outlook

Since the early days of Baeyer–Villiger biooxidations using a handful of crude biocatalysts originating from wild-type organisms with difficult to control enzyme production, the application of BVMOs in synthetic chemistry has matured to a platform technology. Today, a comparably large collection of diverse proteins with complementary substrate specificity and stereoselectivity has become available. In the light of the increasing number of genomes decoded every year and novel technologies for identifying potential BVMOs, there is a clear prospect of discovering new biocatalysts with novel properties and features. In addition, we are starting to understand the architecture of these enzymes based on the structure determination of at least one representative, subsequently allowing for random or knowledge-based optimization of these biocatalysts. BVMOs open up a multitude of appealing transformations for accessing versatile building blocks for asymmetric synthesis. The number of applications in natural product and bioactive compound synthesis is constantly increasing. Still, considering the efficient and facile access to complex scaffolds incorporating several de novo-created stereogenic centers within the biooxygenation process, BVMOs deserve a larger community of users. In particular, in combination with novel fermentation strategies, these enzymes provide access to multigram-scale amounts of valuable intermediates. We consider the recent design of the first self-sufficient fusion protein BVMOs as a very promising contribution by our groups for facilitating the use of this interesting enzyme family further. Due to the simplicity in applying these two-inone second-generation BVMOs in synthetic chemistry, which can even be operated by scientists relatively inexperienced in biochemistry, we hope to proliferate the number of adventurous chemists eager to test some novel and exotic ‘reagents’ to ultimately add biocatalysis to their general portfolio of synthetic methodology.

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References 99 (a) Walton, A.Z. and Stewart, J.D. (2002) Biotechnology Progress, 18, 262–268. (b) Walton, A.Z. and Stewart, J.D. (2004) Biotechnology Progress, 20, 403–411. 100 Simpson, H., Alphand, V. and Furstoss, R. (2001) Journal of Molecular Catalysis B: Enzymatic, 16, 101–108. 101 Doig, S.D., Avenell, P.J., Bird, P.A., Gallati, P., Lander, K.S., Lye, G.J., Wohlgemuth, R. and Woodley, J.M. (2002) Biotechnology Progress, 18, 1039– 1046. 102 Schulz, F., Leca, F., Hollmann, F., Reetz, M.T. and Beilst, J. (2005) Organic Chemistry, 1, 10. 103 Brosa, C., Rodriguez-Santamarta, C., Salva, J. and Barbera, E. (1998) Tetrahedron, 54, 5781–5788. 104 Sometimes called extractive biocatalysis. It was also successfully applied to ketone reduction (a) Vicenzi, J.T., Zmijewski, M.J., Reinhard, M.R., Landen, B.E., Muth, W.L. and Marier, P.G. (1997) Enzyme and Microbial Technology, 20, 494–499. (b) D’Arrigo, P., Lattanzio, M., Fantoni, G.P. and Servi, S. (1998) Tetrahedron: Asymmetry, 9, 4021–4026. (c) Shorrock, V.J., Chartrain, M. and Woodley, J.M. (2004) Tetrahedron, 60, 781–788. 105 (a) Hilker, I., Alphand, V., Wohlgemuth, R. and Furstoss, R. (2004) Advanced Synthesis Catalysis, 346, 203–214. (b) Hilker, I., Gutierrez, M.C., Alphand, V., Wohlgemuth, R. and Furstoss, R. (2004) Organic Letters, 6, 1955–1958. 106 Cells kept at +4 °C were still active after several weeks. Berezina, N., Kozma, E., Furstoss, R. and Alphand, V. (2007) Advanced Synthesis Catalysis, 439, 2049–2053. 107 (a) Hilker, I., Baldwin, C., Alphand, V., Furstoss, R., Woodley, J. and Wohlgemuth, R. (2006) Biotechnology and Bioengineering, 93, 1138–1144. (b) Baldwin, C.V.F. and Woodley, J.M. (2006) Biotechnology and Bioengineering, 95, 362–369. 108 Hilker, I., Wohlgemuth, R., Alphand, V. and Furstoss, R. (2005) Biotechnology and Bioengineering, 92, 702–710.

109 Zambianchi, F., Raimondi, S., Pasta, P., Carrea, G., Gaggero, N. and Woodley, J.M. (2004) Journal of Molecular Catalysis B: Enzymatic, 31, 165–171. 110 Zambianchi, F., Pasta, P., Carrea, G., Colonna, S., Gaggero, N. and Woodley, J. M. (2002) Biotechnology and Bioengineering, 78, 489–496. 111 Abril, O., Ryerson, C.C., Walsh, C. and Whitesides, G.M. (1989) Bioorganic Chemistry, 17, 41–52. 112 Zambianchi, F., Pasta, P., Carrea, G., Colonna, S., Gaggero, N. and Woodley, J. M. (2002) Biotechnology and Bioengineering, 78, 489–496. 113 Atia, K.S. (2005) Radiation Physics and Chemistry, 73, 91–99. 114 Recent reviews: (a) Stahl, S., Hober, S., Nilsson, J., Uhlen, M. and Hygren, P.-A. (2003) Biotechnology and Bioengineering, 27, 95–129. (b) Trejo, F., Gelpi, J.L., Busquets, M. and Cortes, A. (1999) Current Topics in Peptide and Protein Research, 3, 173–180. 115 Examples from the recent literature: (a) Narita, J., Okano, K., Tateno, T., Tanino, T., Sewaki, T., Sung, M.-H., Fukuda, H. and Kondo, A. (2006) Applied Microbiology and Biotechnology, 70, 564–572. (b) Giridhar, P.-H., Wu, R. and Wu, W.-T. (2006) Biotechnology and Bioengineering, 95, 1138–1147. (c) Liu, D., Schmid, R.D. and Rusnak, M. (2006) Applied Microbiology and Biotechnology, 72, 1024–1032. 116 (a) Zhan, Y., Li, S.-Z., Li, J., Pan, X., Cahoon, R.E., Jaworski, J.G., Wang, X., Jez, J.M., Chen, F. and Yu, O. (2006) Journal of the American Chemical Society, 128, 13030–13031. (b) Khang, Y.H., Kim, I.W., Hah, Y.R., Hwangbo, J.H. and Kang, K.K. (2003) Biotechnology and Bioengineering, 82, 480–488. (c) Chen, X., Liu, Z., Wang, J., Fang, J., Fan, H. and Wang, P.G. (2000) Journal of Biological Chemistry, 275, 31594–31600. 117 (a) Metcalf, W.W. and Wolfe, R.S. (1998) Journal of Bacteriology, 180, 5547–5558. (b) Garcia Costas, A.M., White, A.K. and Metcalf, W.W. (2001) Journal of Biological Chemistry, 276, 17429–17436.

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21 Second Generation Baeyer–Villiger Biocatalysts (c) Vrtis, J.M., White, A.K., Metcalf, W.W. and van der Donk, W.A. (2002) Angewandte Chemie, 114, 3391–3393. (d) Vrtis, J.M., White, A.K., Metcalf, W. W. and van der Donk, W.A. (2002) Angewandte Chemie – International Edition, 41, 3257–3259.

118 Woodyer, R.D., van der Donk, W.A. and Zhao, H. (2003) Biochemistry, 42, 11604–11614. 119 Pazmino, D.E.T., Snajdrova, R., Baas, B.-J., Ghobrial, M., Mihovilovic, M.D. and Fraaije, M.W. (2008) Angewandte Chemie – International Edition, 47, 2275–2278.

369

Index a absolute configuration 95 acetate 81ff. acetate ester 81 Acinetobacter 339, 342, 357 acrylonitrile 278, 280, 282 active site 82, 97, 215, 219 activity 67ff., 72, 76 N-acyl amino acids 211 acyl donor 119, 130 acyl donor/alcohol molar ratio 114 acylase 13, 211 additives 73ff. adenosine deaminase (ADA) 145, 147 adenylate deaminase (AMPDA) 145 adsorption 22ff. alanine racemase 217ff. alcohol dehydrogenase (ADH) 2ff. – chiral fluorogenic substrates 2 R-alcohols 99ff., 106 aldol addition 301ff., 305 – stereochemistry 303 aldolase 2ff., 300f. aldolase catalytic antibody 3 aldolase-catalyzed synthesis 299 aliphatic amino acids 190 alkaline protease 57f., 60, 64 alkoxycarbonylation 143 alkylation 10 allosteric effects 93ff., 97 amidase 13, 177, 250ff., 273ff., 276, 284 – kinetic parameters 280 amidasenitrile hydratase 276, 283 – kinetic parameters 280f. amide 257, 274, 284 amide formation 268f. amidinomycin 249 amino acid 318 α-amino acid 199, 205f.

– deracemization by stereoinversion 202ff. – deracemization via DKR 205 β-amino acid 247f. γ-amino acid 256 D-amino acid 173, 187ff. – production 173 L-amino acid deaminase 217 amino acid esters 210 amino acid oxidase 212f., 318 D-amino acid oxidase 202ff., 213ff. – classification 214 L-amino acid oxidase 216f. amino acid racemase 204, 217ff. – PLP-dependent racemases 217ff. – PLP-independent racemases 220 amino aldehyde 303 γ-amino butyric acid (GABA) 248f. β-amino nitrile 250f. – enantioselective hydrolysis 250, 254 γ-amino nitrile 255ff. – enantioselective hydrolysis 256 amino protecting group 250 amino transferase 221 – D-amino transferase 222f. – L-amino transferase 202ff., 222f. aminocyclitol synthesis 291f. aminonucleoside 142 anomeric separation 143 antibiotics 173, 188, 215 anti-HBV drug 136 anti-HIV drug 136 apoptosis 216 aromatic amino acids 190 Arrhenius plot 83, 277f. aspartate racemase 186, 220 Aspergillus aculeatus 153ff., 157ff., 168 Aspergillus niger 229f., 235ff. assay 1ff.


370

Index asymmetric dihydroxylation 316, 325 asymmetric oxidative transformations 330 asymmetrization 103ff.

b Bacillus cepacia lipase (BCL) 33f., 37 Baeyer-Villiger biocatalyst 339ff. Baeyer-Villiger monoxygenase (BVMO) 15, 323, 328, 339f., 346f., 353 – application 363 – engineering 342ff. – enzyme platform 341f. – functional group selectivity 350 – immobilization 361 – phylogenetic relationships 344 – stereoselective sulfoxidations 357 Baeyer-Villiger oxidation 315, 321, 323f., 331, 347ff. benzonitrile 276f., 284 bicycloketone 353 bienzymatic cascade 264ff. bioactive compound synthesis 354f. biocatalysis 21, 64, 67f., 114f., 123, 135ff. biocatalyst 21ff., 55, 57, 88f., 115, 120, 149, 163, 263, 300 biocatalytic asymmetric oxidation 313ff. – historical industrial examples 316 – with dehydrogenases 320 – with dioxygenases 325ff. – with monooxygenases 321ff. – with other enzymes 328ff. – with oxidases 317ff. – with peroxidases 319f. biocatalytic oxidative cyclization reactions 329 biocatalytic process intensification 50f. bioconversion 203 biodegradation 273 biooxygenation 352 bioreactor 51, 274 biotransformation 1ff., 79f., 109, 135, 250ff., 274, 276f., 352, 355f., 359 N-Boc-amino acid-thioester 209

c C-C bond formation 287ff. Candida antarctica lipase B (CALB) 21, 23ff., 35, 68f., 71ff., 98ff., 111f., 116, 125, 137, 145 carbamoylase 177, 188, 206f. carbohydrate 287 carbohydrate fatty acid ester 56ff. carrier 22f., 25, 242

catalyst 21f. – heterogeneous catalysts 22 – homogeneous catalysts 22 celite 26, 70 cellubiose 56 central nervous system (CNS) 248f. chemoenzymatic modification 136ff. chemoenzymatic synthesis 287 chemoselectivity 109, 347ff., 355 chiral 93, 110, 195 chloroperoxidase 28ff., 320 circular dichroism 60 clips-O substrate 8, 11ff., 14f. clopidogrel 263 co-enzyme regenerating enzyme (CRE) 362 co-factor 341 co-lyophilization 69ff., 73 competitive inhibition 82 conformation 59ff. continuous-flow microchannel reactor 43ff. continuous stirred UF-membrane reactor (CSMR) 274ff., 279, 281ff. – CSMR investigation 277f. conversion-time diagram 256f. copper calcein 14 covalent binding 33, 35ff. cross-linked enzyme aggregate (CLEA) 35, 37f., 265 cross-linking 33, 35ff. cyanide dihydratase 230, 234f. cyanohydrin 9, 24, 201, 264, 269 – nitrilase-mediated hydrolysis 262 3-cyanopyridine 278, 280f. cyclodextrins 59 cyclohexanone monooxygenase (CHMO) 324, 342f., 345ff., 357, 362 – dynamic kinetic resolution 351 cyclooxygenase (COX) 86, 326 cyclopentanone monooxygenase (CPMO) 343, 345f., 353

d deaminase 145f., 149 deamination 146, 216 dehydrogenase 320, 325 deracemization 195ff. – strategies 195f. – useful enzymes 213ff. desymmetrization 356f. DHAP synthesis 288ff. – from dihydroxyacetone 289 – from Rac-glycidol 290 N,N′-dialkylimidazolium cation 116

Index diastereoisomer 304f., 308 diasteroselectivity 252, 304, 307, 309 dihydrocarvone 354 dihydroxyacetone (DHA) 289f., 301, 308f. dihydroxyacetonephosphate (DHAP) 287, 289ff., 299f., 301, 303, 309 dioxygenase 315, 325ff. directed evolution 342 disaccharides 56f. dopamine hydroxylase 323 dynamic kinetic resolution (DKR) 350f.

e E-value 84, 94ff., 106 – influence of enzyme immobilization 97 – influence of the reaction medium 96 electrostatic interactions 22ff., 27 enantiomer 94f., 173, 195 enantiomeric excess (e.e.) 93f., 96, 119, 197, 210, 252f. – during asymmetrization reactions 105 enantiomeric ratio see E-value enantiopure compound 93ff., 101, 106, 261 enantiopurity 253 enantioselective activation 97f. enantioselective inhibition 97f. enantioselective synthesis 261ff. enantioselectivity 5, 25, 32, 38, 60, 64, 67ff., 71f., 76, 87, 93ff., 116, 120, 139, 197, 248, 263, 266, 296, 343, 348 enantiospecific oxidation 318f. encapsulation 31ff. enolase 4ff. enzymatic acylation 128 enzymatic separation 144 enzyme 55, 61f., 361 enzyme catalysis 202, 210, 214, 219f., 223 enzyme-catalyzed asymmetric oxidations 331 enzyme engineering 15 enzyme formulation 67ff. enzyme immobilization 21ff., 47f., 97 – effect of pH and ionic strength 162f. enzyme inhibition 97 enzyme recycling 24 enzyme stability 59ff. epoxidation 321 epoxide hydrolase 11ff. ester 9, 79ff. esterase 5ff. esterification 55, 79ff., 89, 99, 101 experiment design 275

f fermentation up-scaling 358 filamentous fungi 229ff. fingerprint 16 flavin 341 flavoenzyme 216 flavolignan 125, 127 flow rate 282f. fluorescence 1 fluorescence assay 1ff. fluorescence lipase assay 7 fluorogenic acyloxymethyl ether 10 flurbiprofen 86f. Fourier-transform infrared (FTIR) spectroscopy 59f., 67, 73 FRET (fluorescence or Förster resonance energy transfer) 11 FRET-lipase probes 11f. β-fructofuranosidase 159 fructooligosaccharide 154 – batch production 167 fructooligosaccharide synthesis 153ff., 159, 164ff. fructose-1,6-bisphosphate aldolase (FruA) 287f. D-fructose-1,6-diphosphate aldolase from rabbit muscle (RAMA) 301ff. D-fructose-6-phosphate aldolase 307f. – isoenzymes 308 fructosyltransferase 153ff., 157ff., 168 – effect of pH and temperature 158 – immobilization 161ff. – influence of chemicals 158 – kinetic behavior 159 – purification 155ff. – substrate specificity 157f. Fusarium solani 229, 234ff.

g galactooligosaccharide (GOS) 155 gas phase microreactor (GPMR) 46ff. gel filtration chromatography 156 glutamate racemase 186, 220 glutathione reductase family 214 L-glycerophosphate oxidase (L-GPO) 290 glycoconjugate 299 glycosidase 6, 299 glycoside hydrolase family (GH family) 153 glycosyltransferase 299 – classification 153

h 5-halo-D-xylulose 293 hepatitis B virus (HBV) 135

371

372

Index heteroatom dealkylation 321f. heteroatom oxidation 321 high-pressure liquid chromatography (HPLC) 46f., 88, 125, 159, 166, 180 high-throughput screening assay 5 histidine 30 human immunodeficiency virus type 1 (HIV-1) 135 hybrid antioxidant 130f. hydantoin 175f. hydantoin racemase 173ff. – activity 182 – amino acid sequences 178, 183f. – binding experiments 186 – biochemical characterization 180 – kinetic analysis 181f. – kinetic parameters 183 – molecular characterization 175 – phylogenetic analysis of amino acid sequences 179 – reaction mechanism 183, 187 – reaction profile of the enzymatic racemization 181 – search for new hydantoin racemases 175 – substrate enantioselectivity 181f. – substrate inhibiton 182 hydantoinase 173, 188 hydantoinase-carbamoylase system 206ff. – D-amino acid synthesis 206 – L-amino acid synthesis 207 hydantoinase process 173, 177, 187, 207 hydratation 268f. hydrazine 140 hydrocyanation 265f. hydrogen bonds 25 hydrogen peroxide 314, 319 hydrolase 11ff., 69f., 106 hydrolysis 27, 139, 144, 265f., 269 hydrophilic carrier 26f. hydrophilic solvent 64 hydrophilicity 63 hydrophobic carrier 28 hydroxy acid 196 – deracemization by DKR 197f. – deracemization by microbial stereoinversion 200f. – deracemization by stereoinversion 199f. hydroxyacetophenone monooxygenase (HAPMO) 343, 357f. α-hydroxycarboxylic acid 261ff. β-hydroxyester 197 hydroxyketone 9 hydroxylation 321

α-hydroxynitriles – deracemization 201f. hydroxynitrile lyase (HNL) 26, 32, 37, 201, 264, 267 hydroxypyruvate (HPA) 288, 294

i ILs/SCFs biphasic system 117 imino compound 212 iminosugar 299ff. – chemo-enzymatic synthesis 300 – DHAP-aldolase-mediated synthesis 301ff. immobilization carrier 161f. in situ base-catalyzed racemization 208 in situ racemization 205f., 209ff. in situ reduction 212 ionic interactions 27ff. ionic liquids (ILs) 64f., 109ff., 114ff., 124, 131f. – IL anions 115 – IL cations 115 – properties and behavior 115 isoenzyme 38 isomerization 295 isopenicillin N synthase 328

k kcat 82, 84 kinetic characterization 273ff. kinetic resolution 94f., 109ff., 113, 120, 206, 208ff., 350

l lactose 48ff., 56 – continuous conversion 48ff. – enzymatic hydrolysis 48ff. L-leucine dehydrogenase 204 Lineweaver-Burk plot 98 lipase 5ff., 23, 26, 55f., 67ff., 80 – activity 73ff. – enantioselectivity 73ff. lipase formulations 68 – activity 68 – enantioselectivity 68 lipase substrate 6 lipophilicity 123 lipophilization 123ff. lyophilization 89

m maltose 56 mandelate racemase 198, 221 mandelic acid 198, 266

Index mass spectroscopy (MS) 125 methyltrimethoxysilane (MTMS) 32f. Michaelis constant (Km) 82, 84 Michaelis-Menten equation 98 micro-array 6f. microbial stereoinversion 200 microchannel 43ff., 47 microchannel reactor 44, 50 microfluidic immobilized enzyme reactor 45 microorganism 229 microreaction technology 44, 50f. microreactor 44ff., 51 microstructure 43 molasses 164ff. mold 79ff., 91 monooxygenase 315, 321, 349 monosaccharide 296 monosaccharide analog 287ff., 296 monosyccharide oxidase 319 mycelia 79ff., 88f., 91

n N-protecting groups 304f. natural antioxidants 124 – modification in ionic liquid media 128ff. – modification in organic solvents 126f. neurotransmitter 323 nicotinamide 278, 280f. nicotinic acid 278 nipecotic acid 255 nitrilase 229ff., 247ff., 261ff., 273 – activity 240ff. – alignment of amino acid sequences 231 – catalytic properties 236ff. – classification 238 – co-purification 234 – distribution and evolutionary relationship 230ff. – electron micrographs 236 – expression 233 – homology analysis of amino acid sequences 232 – immobilization 242 – induction 243 – mechanism 270 – molecular genetic analysis 230 – molecular masses 235 – nitrilase biotransformation 254 – nitrilase inducer 233 – nitrilase genes 242f. – nitrile hydratase activity 257 – prokaryotic nitrilases 239

– reaction mechanism 236f. – reaction optima 241 – selection and screening of nitrilase activity 232f. – selectivity 262 – stability 240ff. – structural properties 234 – substrate specificity 238f. – temperature optima 240 nitrile 238, 247, 263, 269, 274, 284 nitrile hydratase 247ff., 267ff., 273ff. nitrile hydratase-amidase cascade system 275ff. – temperature dependence 275f. nitrile hydrolysis 247 nitrocyclitol 291 nitrocyclization 292 non-competitive inhibition 98 non-conventional media 109ff. non-proteinogenic amino acids 247ff. non-steroidal anti-inflammatory drug (NSAID) 86 novozym 435 24f., 27, 35, 101f., 111f., 125 nuclear magnetic resonance (NMR) 125, 305 nucleoside analog 135ff., 148f. nucleoside synthesis 147ff.

o oligosaccharide 299 organic peroxides 314 organic solvent 61, 67ff., 73ff., 111, 124, 127, 132 oxidant 314 oxidase 317ff. – synthetic applications 317 oxidation reactions 313ff. oxidative conversion 339 oxidative decarboxylation reactions 315 oxidative phosphorylation 315 oxidoreductase 316 oxygen 313ff.

p parallel kinetic resolution (PKR) 143 partition phenomena 88ff. penicillin G amidase 13 periodate 8 peroxidase 319f. phenolic antioxidants 123ff. phenyl oxyzolinone 208 phenylacetone monooxygenase (PAMO) 341, 357f., 362

373

374

Index – active site 345 – crystal structure 347 phosphatase 14f. phosphite dehydrogenase (PTDH) 362 pipecolic acid 212 pipecolic amide 255 piperidine 306 polyacrylamide gel electrophoresis (PAGE) 155f. polydimethylsiloxane (PDMS) 44, 46f., 49f. polyethylenimine (PEI) 28 polyhydroxylated compounds 128f. polymerase chain reaction (PCR) 177 porosity 163 pressure 112f. prochiral compounds 103ff., 107 prochiral dicarboxylates – asymmetrization 103ff. prochiral diol – asymmetrization 105 protease 13, 55ff. – activity 55ff. – regioselectivity 56 – secondary structural elements composition 61 – stability 55ff. protection-deprotection 314, 319f. protein-coated micro-crystal (PCMC) 30f. protein-protein interaction 76 Pseudomonas cepacea lipase, free (PSL) 137, 143 Pseudomonas cepacea lipase, immobilized (PSLC) 137, 142ff. pyrrolidine 306

r racemate 195 racemic alcohols 83ff. racemic carboxylic acids 85ff. racemic mixtures 94 racemization 174ff. – chemical racemization 174, 187 – enzymatic racemization 175 reaction media 301, 303 reaction rate 103, 279 reactor capacity 279 RebC 331 RebP 331 recombinant biocatalyst 187ff. reductive amination 306, 309 regiodivergent biooxidation 353f. regioselectivity 59, 93, 109, 129, 132, 138f., 144, 351ff. relative humidity (RH) 88ff.

resin 359f. reverse-phase high-performance liquid chromatography (RP-HPLC) 250 Rhodococcus 251 ruthenium catalyst 197f. rutin 124, 126

s Schotten-Baumann reaction 211 screening 79ff. sec-alcohol 109, 120, 199 selectivity 38 self-sufficient fusion protein BVMOs 361 sepabeads EC-EP 161ff. serine racemase 219 silybin 124, 126 site-directed mutagenesis 218 sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) 180 sol gel technique 31f. solvent engineering 63f. solvents 55ff. space-time yield (STY) 51 space velocity 282f. stereoinversion 195f., 199f. stereoselective esterification 83ff. – racemic alcohols 83ff. – racemic carboxylic acids 85ff. stereoselective sulfoxidation 357 stereoselectivity 4, 109, 304f., 351ff., 363 styrene monooxygenase 324 substrate 76 substrate conversion 279 substrate feeding product removal method (SFPR) – in situ SFPR 359 – resin-based SFPR 360 subtilisin 57f., 60, 70 sucrose 56 sugar beet syrup 164ff. supercritical carbon dioxide (SC-CO2) 110ff., 116f. supercritical fluids (SCFs) 109, 111f. surface tension 113 synthesis 135ff. synthetic chemistry 347ff.

t tetrahydrofuran 356 tetramethoxylsilane (TMOS) 31, 33 thermostability 82 thiamine pyrophosphate (ThDP) 288 thin layer chromatography (TLC) 250 5-thio-D-xylopyranose 293ff.

Index transaldolase 4 transaminase see amino transferase transesterification 100ff., 131 transesterification activity 69ff. transformation 146f., 251f., 339 transglycosidase 153 transglycosylation 147ff., 160 transketolase 4, 287f. – activity assay 295 – modification of substrate specificity 295f.

u UF-membrane bioreactor 273ff. umbelliferone 5f., 10

v valiolamine analog 292 van der Waals interactions 23ff. vancomycin resistance 219 viscosity 113

w whole cells 358

x x-ray crystallography 218 D-xylopyranoside 294 5-D-xylulose 293ff. 5-D-xylulose analog 293ff.

375

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